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#804 Geeky details about the "Normals Visualization" available in the new...

Posted by Carla Schroer on 15 January 2014 - 09:56 PM

The 1.1 RTiViewer has the ability to display a normals visualization.  There is some basic information about this in the User Guide on page 23. Additionally, the concept of surface normals and how they are calculated by having known light positions is described on the CHI website on the RTI page:   However, we do receive questions about exactly what the normals visualization represents and how the data is stored.  Here is a geekier explanation than is in either of the above places for those that want to look under the covers.


Background - The surface normal is a vector that is perpendicular to the tangent plane for any point along a 3D surface.  In RTI files, the surface normal is calculated per pixel based on the image capture set and on knowing the light position for each image in the capture set.  Surface normal accuracy can be affected by: shadows and highlights on the subject; sample size and spread of the lights in the image capture set; accuracy of the light position calculation; and which algorithm is used to calculate the normals. Other factors include whether the images are aligned, and whether the images are in focus for the areas of the subject from which you want to calculate normals.


Surface normal calculation and representation - The surface normal calculation is part of the fitting algorithm, though the data in a PTM or RTI file are not stored directly as surface normals.  When a PTM or RTI file is loaded into an RTI viewer, it calculates the surface normals for use in the viewing environment.  The surface normal is represented by x, y, and z coordinates, which are calculated as floating point numbers, normalized to be between -1 and 1 (in addition, x*x + y*y + z*z = 1; in other words, length of a normal vector has to be 1.) The x, y and z coordinates correspond to a point where the origin (0,0,0) is on the surface at that pixel, and the x,y,z coordinates describe a point in space away from the origin.  The normal vector starts at the surface and goes through that point.


Representation of normal fields - It is common to represent normal fields through false color visualization, where the x, y, and z coordinates are mapped to RGB: red, green and blue, respectively.  The normal visualizations are useful in their own right, as well as carrying the coordinates that describe each normal per pixel (as described above). This means that the numerical data for the normals can be compared for a variety of purposes, such as tracking changes to the surface of the same object, or comparing similar materials, for example in a study of tool marks. There are some alignment issues to resolve.  Once resolved, the data can be compared numerically.  This opens up a variety of additional studies beyond just visual inspection in a viewing environment.  (Note there are solutions for alignment, but that topic goes beyond the scope of this post)


The attached image shows the surface normals visualization applied to a hemisphere.Attached File  18-normal.jpg   146.54KB   8 downloads


I want to stress that, because various factors can affect normal accuracy (as described above), using normal comparisons for study requires capturing high-quality RTI data and the application of the HSH algorithm (or new algorithms as they become available).


I would like to thank to Sema Berkiten, PhD student at Princeton, for walking me through the details of this.

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#1487 RTI Dome Build Instructions

Posted by leszekp on 21 June 2016 - 05:56 AM

I've started a Hackaday page on how to build an RTI dome like the ones I've used for this work on lithic artifacts. It can drive 3W/1A LEDs, so should be able to light up a dome at least a meter in diameter (maybe even bigger, depending on how long an exposure time you're willing to tolerate). Still in the early stages, but I've got a fairly complete parts list up, along with some background on the design; instructions should start going up shortly. Comments and suggestions welcome. Open Hardware license, Creative Commons on the instructions, software when released will be under GPL v. 2.0.

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#1221 RTI For Lithic Artifacts

Posted by leszekp on 06 May 2015 - 07:39 AM

About 3 weeks ago, I did a presentation at the Society For American Archaeology conference in San Francisco, titled "Documentation Of Lithic Artifacts Using An Inexpensive Reflectance Transformation Imaging System". I've just put an extended video version of that talk online at a website I've put together. It covers not just lithic artifact imaging with RTI, but also a field-ready portable RTI system I designed and constructed, and some experiments with converting RTI data into 3D models. The site also contains links to both RTI data files and 3D ply files for lithic artifacts, to view and download. Hope you find it interesting.

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#868 Spheres in corner of frame, distortion, poor ptm

Posted by marlin on 11 March 2014 - 05:22 PM

Hello mgts24!


thanks for the message. I will attempt to reply to your post with some useful information.


You have experienced Lens Distortion. The 24mm-105mm L is a really nice lens, but it may not be the best lens to collect RTI data sets.


As we know, Lens Distortion is in every lens -- thats just optics.  Some have more, some have less, a lot less. We steer professional towards the prime lenses.



The 50mm Macro Prime is one of our daily drivers in our camera bag:



The other worker bee is the 100mm L (or its previous)




Both of the above lenses have the least amount of lens distortion, or rather, the most acceptable amount of lens distortion.


Lens Resource

Websites like http://www.dpreview.com/ (go here and look up your lens), can offer valuable insight into the distortion(s) that are inherent of the lens in your camera bag.


For example check out this interactive chart on sharpness on the 100mm Macro L. There other good stuff in there too. (note the comments about the 100mm being exceptional in the distortion category)




A word about zoom lenses (lenses which are adjustable, e.g. : 100mm-400mm). Its not bad to collect RTI data with a zoom lens, but its a lot better to use a Prime lens --- we know this. But another consideration, is that zoomable lenses, lenses that have that turnable ring to adjust your FoV, will often times *shift - especially if the camera is pointed down towards the earth. Your very first image might be tack sharp and in focus, but then the last image in the RTI data set might be soft and out of focus. I've seen this numerous times. The lens simply shifted during the capture sequence. If you have to use a lens that translates to adjust the FoV, then use 'tape' (gaffers tape) to physically 'tape' the lens in place to itself ---- preventing movement and keeping all silent. (don't forget that you also Must be on Manual Focus "MF" mode).


Spheres in the corner.

Spheres in the corner of the frame will appear egg like with lenses that contain a lot of lens distortion. Even the best prime lenses might have a little bit of 'wobble' to them. Moving the spheres towards the middle of the frame even just a tiny bit can help the distortion. In some setups you can control the sphere placement, if you can do this, you might as well. Other times, bc of the subject the sphere may end up in the corner. Remember that RTI Builder is pretty flexible and will most times, more than not, still offer a usable and acceptable result ---- even if the sphere is egg like. (hopefully enough to reveal that hidden text you are looking for).


Lens Distortion Correction.

quick word on Lens Distortion correction. If you want to correct for lens distortion, there are a few ways to do this. Its too much to disscuss in this post but, if you do correct your images, you would want to correct them at the DNG level or at the 'jpeg-exports' phase, and then continue to process your data from there.


Hope that this Helps.

Happy F-stop.



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#847 DIY LED RTI array

Posted by Charles Walbridge on 26 February 2014 - 04:52 PM

I've put together a cheap and quick lighting array for making RTIs of daguerreotypes we have in the collection of the Minneapolis Institute of Arts. I used lighting equipment we have in the photo studio, LED spots we use in the galleries, and about $30 worth of lamp wire and cord switches. 

The light stand I've used has wheels, so now two of us can shoot the 48 source images for an RTI in about 10 minutes. Because the four lights on the array are the same distance from the object, I can position the array at 12 o'clock, measure the distance to the object with my fancy RTI string, and then turn the lights on and off from the cord switches on the individual lamps. Then I'll roll the stand to the one o'clock position and repeat the process.

I've put up pictures of the array on a Google Plus page here:


and I can share them with the RTI community on Facebook too.

Let me know what you think --



Attached Files

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#780 RTIViewer 1.1 release now available

Posted by Carla Schroer on 05 December 2013 - 07:11 AM

We are thrilled to announce the release of RTIViewer 1.1.  The new features in this release are the ones most requested by our users.


You can read about the new features and download the release (windows and mac versions), user guide and examples on the RTIViewer page.


This software is available as free open source software.


Cultural Heritage Imaging is a small independent nonprofit organization and we need donations from people like you who use and value these tools in order to keep doing the releases.  Please help us by making a donation to support this work.


We want to gratefully acknowledge the efforts of Ron Bourret, Gianpaolo Palma, and Leif Isaksen for the development work that went into this release.  Much of the work was performaed as volunteers.  We also want to thank Judy Bogart for doing the User Guide updates and descriptions for the new features. We had some beta testers for the release, and we thank them for their time and effort to try things out and report issues. And finally, we thank all the members of the CHI team for the work to oversee the release including setting the requirements for the release, testing it and providing feedback in every phase of development, running the beta testing program, identifying and reviewing the user guide updates, and all the other tasks that go with getting something like this done and out.  Much of that work was done as volunter labor as well.


If you have comments or questions, or want to say nice things about the new release, please post those in the "All Viewers" forum





Attached File  rti-viewer-interface-sm.jpg   209.88KB   0 downloads

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#728 Minimum reflective sphere diameter and HSH precision

Posted by Mark Mudge on 20 August 2013 - 05:41 AM

Here is the reasoning behind CHI's recommendation that there be at least 250 pixels along the diameter of the reflective spheres in an RTI image. The RTIBuilder software finds the pixel in the exact center of the highlight produced by the illumination source. The more pixels there are across a reflective sphere, the more the incident illumination direction can be refined.

If you look across a reflective black sphere (or any shiny sphere), the middle part of the sphere reflects the hemisphere of light in the direction of the camera. The outer part of the sphere reflects the hemisphere behind the reflective black sphere. This is how a sphere, often called a light probe, can capture the illumination information of an entire environment.

When building an RTI we are only concerned with the central part of the reflective black sphere because the light positions used to illuminate the subject exist in the hemisphere facing the camera. The number of pixels in the diameter of this central region of the reflective black sphere determines the angular resolution of the incident light direction. If there are 180 pixels across this central region the angular resolution will be 1°. If there are 90 pixels across the central region the angular resolution will be 2° and so forth. CHI recommends 250 pixels across the entire reflective black sphere because that will ensure that there will be at least 180 pixels across the central region of the reflective black sphere that is used in calculating RTI incident light positions (which are stored as x.y.z coordinates in a light position file - which is calculated from the highlight data)  For an explanation of how the light positions are calculated from the highlight data, see our paper with Tom Malzbender from VAST 2006.

If your purpose is to use your RTI for visual interpretive purposes, there will likely be no perceptible differences between an angular resolution of 1° and 2°. If however your purpose was to refine or generate a three-dimensional surface, the angular resolution of the incident light source and the resulting surface normals contribute significantly to the accuracy of the 3-D surface. As we cannot foresee how the documentation we produce today will be re-purposed by others in the future, we suggest, when practical, to capture the highest quality data.



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#657 RTImeline

Posted by Graeme Earl on 30 June 2013 - 11:43 AM

Hi all,
I am giving a talk in York, UK on Saturday 6 July 2013 at Digital Heritage 2013: Interfaces with the Past.
As part of that I thought I might create an MIT SIMILE "RTImeline" charting the development of RTI, and its components. (I don't think this exists already - of course if it does please let me know.)
I realise this might quickly dissolve into disagreement (!) but do people have some ideas of key events in the development of RTI that should go on the timeline? If so respond to this with:
- title
- one or more URLs if possible
- a date
- an end date if you want to specify a range
I realise this is cheeky as I am the one giving the presentation but of course all contributions will be very gladly acknowledged!
Some uncontroversial examples (I think - please let me know if I am wrong!):
Publication of "Enhancement of Shape Perception by Surface Reflectance Transformation" by Tom Malzbender, Dan Gelb, Hans Wolters and Bruce Zuckerman
March 2000
Publication of "Polynomial Texture Maps" by Tom Malzbender, Dan Gelb and Hans Wolters
August 2001




Publication of first on-line PTM viewer by Clifford Lyon.




December 2004



Publication of "Surface enhancement using real-time photometric stereo and reflectance transformation" by Tom Malzbender, Bennett Wilburn, Dan Gelb and Bill Ambrisco
June 2006
Publication of "New Reflection Transformation Imaging Methods for Rock Art and Multiple-Viewpoint Display" by Mark Mudge, Tom Malzbender, Carla Schroer and Marlin Lum. First us of Reflectance Transformation Imaging. Introduction of Highlight RTI and PTM Object Movies,
November 2006


Publication of "Image-Based Empirical Information Acquisition, Scientific Reliability, and Long-Term Digital Preservation for the Natural Sciences and Cultural Heritage" by Mark Mudge, Tom Malzbender, Alan Chalmers, Roberto Scopigno, James Davis, Oliver Wang, Prabath Gunawardane, Michael Ashley, Martin Doerr, Alberto Proenca and João Barbosa. Introduciton of Empirical Provenance in the context of RTI


April 2008




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#337 Introduction to the project

Posted by Graeme Earl on 19 February 2013 - 12:46 AM

The UK Arts and Humanities Research Council (AHRC) have provided follow on funding to support the development of an on-line, open source RTI viewer that works on all platforms. We would welcome all input to the project and we will be sharing all of our progress as the project develops. More details to follow ASAP.

There is a round-up of the activities from the previous project here: http://acrg.soton.ac.uk/tag/rtisad/

You can follow the project via @AHRCRTI and my input via @GraemeEarl @AHRCRTI


Note concerning privacy:


Any content that is posted on this forum is public and will get indexed by search engines. You have to make an account to post here and to see the list of members. There is information about individual members available to other members, but only if you choose to fill in profile information for your account. Only administrators on the system see the member's email addresses.

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#1164 HSH or PTM - How to choose the best fitter

Posted by James Davis on 20 February 2015 - 07:04 PM

You can think of this as a sparse data interpolation problem. We measure N lighting directions, and we have to fit M polynomial coefficients. The data is noisy, not properly bandpass filtered, has outliers, etc. If you think back to your freshman calculus or other numerical class, in an ideal world you need at least M knowns, to estimate M unknowns, so we have N>M. If this isn't true, then we have an overfitting problem. Of course the data isn't perfect, so as a rule of thumb, lets say we need (N/2)>M. 


It was asked above what overfitting is? See the image below for a visual/math way to think of this problem. In practice, it means that you will see "noise" appear when you put the lighting direction at any direction other than the ones you sampled. This is the extra wiggling in the plot on the right that isn't real data. This happens when you have too many polynomial terms.




Now on the question of PTM vs HSH, what are we changing, we are changing the choice of polynomial. They are both polynomials, but maybe one as a term for xy and the other has a term for x^2. The original PTM paper defined 6 terms. The plot given above from the Zhang et al paper has a definition that allows a variable number of terms. This is the first time I've seen PTM defined with a variable number of terms, and I think all existing fitters and viewers use the 6 term definition. HSH are Hemi-spherical Harmonics. They are also polynomials, but have a historical mathematical definition which includes 4 terms, 9 terms, 16 terms, etc. The question is which polynomial terms are best? Well, that depends on your data. We did some experiments in the 2007 time range of just trying random polynomial terms to see if we could do better than PTM or HSH, and indeed we could, but it was dependent on the images we tested, and we abandoned that research before finding a new set of terms which was always better. The plot above says that for whichever set of images was tested, the extended definition of PTM was better than HSH. 


In terms of real choices of tools you can use, you can have PTM-6 or HSH-4, HSH-9, HSH-16, there aren't widely available tools for anything else. Since we dont really have compression built into any of the tools, you can expect the file sizes to roughly scale with the number of terms. You can also expect to fit the data better with more terms. Since matte surfaces are more flat and specular surfaces have a bump at the highlight, then thinking about the plot above, we can see that more terms let us represent the bump of the specular highlight better.


The last point is about how to evaluate error. In papers we like our nice plots. We generally use some metric that comes down to a number. It might be RSME or some perceptually driven metric, but it always comes down to a "quality number". This is a gross simplification. In practice none of these methods know what the object *really* looks like between light directions, because we didnt capture an image there. So we are making up what it looks like. Its the space between the samples in the plot above, is it straight?  or curved? or has a wiggle? We just dont know. It might be true that the "quality number" thinks the wiggle has lowest error, but viewer A likes the straight fit and viewer B likes the curved fit. This is why I said earlier in this thread that you would have to look and see what you like. In the image processing papers people often use Structual Simimlarity (SSIM) when they want a human perceptual number, but its still just a "quality number" which still grossly simplifies the situation. In my experience its primarily a feel good for researchers to claim they are doing the right thing, but its not substantially different than RSME.



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#910 A post of RTI-HSH stills of lead-soap microprotrusions on Google+

Posted by GOKConservator on 28 March 2014 - 11:01 PM

A set of RTI still images of typical lead-soap micro-protrusions seen in commerically primed Belgian canvases used by O'Keeffe are now posted on my Google+ page: https://plus.google....ronkright/posts Color, specular and normal vector visualization. Enjoy!

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#785 RTI in the Minneapolis Institute of Arts' iPad magazine

Posted by Charles Walbridge on 19 December 2013 - 09:03 PM

Hey all - I'm pleased to tell you that our article featuring RTI is on the cover of the December 2013 issue of Verso, the Minneapolis Institute of Arts' free iPad magazine.

It's here in the iTunes Store:


The web version of the magazine, which has most of the functionality, is here:


I'm pretty pleased with it. Our curator Roberta Bartoli has asked me to present RTI to her Art History students, and last week we showed the technique to some of the museum's trustees.




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#662 Screen Recording software ... (Macs)

Posted by Dennis on 02 July 2013 - 11:42 AM


Thanks for the post on Screen Flow. It looks great. For those of us for whom even the low cost ($99) of Screen Flow is a stretch I recommend the free version of a utility called Screen Record Utility Lite Version 2.0, downloadable from the App Store. It gives the Apple Mac user the ability to make movies of their RTIviewer files lasting up to 300 seconds. For $4.99 that maximum time limit is removed but five minutes of viewing a single interactive RTI is a long time! The recordings are saved in .mov format at up to 30fps without a watermark.


From my tests of the demo version of Screenflow and the free Screen Record Utility Lite it's clear one gets value, primarily video editing and .mp4 formatting, for the $99. However the free utility does give quick Powerpoint insertable movies of RTIs that run and look fine.




iMac 2011


PPT 2011 v14.3.5

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#627 RTI Underwater: a research project. University of Southampton

Posted by David Selmo on 07 June 2013 - 10:57 AM

I have completed the prototype of the first 'Underwater RTI Light Dome'


A HUGE thank you to Professor Mark Jones,  Master carpenter Mr. Dennis Cook,  Simon, and my new friends at the Mary Rose Museum in Portsmouth, UK for allowing me to build this in their workshop!


Pictures of it in the Google Drive folder:




It was made entirely of acrylic cut with a laser.  It essentially has a focal length of 400 mm (inside radius), and a total base ring OD of 880 mm.  Each leg features position for four LEDs.  It will hold both my $600 'point and shoot' auto-focusing digital camera and the department's $8000 worth of high-end underwater Nikon SLR equipment. I designed it to be completely modular, although to make it firm enough would have required additional bracketing.  I ran out of time at the workshop to make these additional brackets.  So unfortunately, it will be completely fused together with acrylic solvent/glue prior to wiring with LEDs.  


(@ George:  I didn't fully understand what you meant by 'surface normal visualization.'  Somebody showed me the RTIBuilder codes to display it and we took a look though. One of my PhD student advisors seemed to think it was due to shadows/reflectivity of the pool surface being tile.  ??  maybe?)


@ Marlin.. the CHI RTI kit arrived!  Thank you.  Now to start modifying the reflector balls so I can place them on target in the open sea.  I'm thinking just gluing a nut on to receive a piece of threaded rod for each.  Then I can just push the rod down into the sand/silt at the same relative height as the object to be photographed.)

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#58 Welcome!

Posted by Carla Schroer on 09 July 2012 - 04:56 PM

This forum is intended for the community of users, developers and others interested in Reflectance Transformation Imaging (RTI), Algorithmic Rendering, and related computational photography techniques.

This forum is hosted and maintained by Cultural Heritage Imaging (CHI) (http://culturalheritagimaging.org) as a public service. In order to post in the forum, you must sign up for and verify an account, but there are no fees or other requirements for membership. Except for the Announcements section, posts are not moderated.

CHI staff will monitor the forums, and warn users who post SPAM. We reserve the right to inactivate accounts if abuse continues.

You can see a short description of the existing boards here:

We welcome suggestions for additional areas.
If you have private comments or suggestions, send email to: forums@c-h-i.org

Thank you!

The CHI team
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#431 Photogrammetry

Posted by Carla Schroer on 12 April 2013 - 08:55 PM

Hi Folks,


I think there have been some tests like this done, and I was trying to find links.  Of course, it never hurts to do your own experiments.


Here are a couple of photogrammetry resources, I'll throw out there.  They don't answer the specific question, but might be of use.


First - the tem at the Georgia O'Keefe museum did a summer project last summer where they used both RTI and photogrammetry to document a variety of material.  They shot some great how-to videos, and they also did some videos discussing their experience and what was easy and what was hard. etc.  They used Agisoft for processing the photogrammetry, and they also posted a couple of video tutorials of their workflow.  They put up a blog site with all this material.  It can be a bit hard to find things on this site, but if you poke around you will find things. A bunch of the vides are on this page: http://okeeffeimagin...umenting/video/


There were a number of talks at the NCPTT sponsored 3D summit held in San Francisco last summer.  They have been putting up the videos of the talks a few at a time.  Here's an interesting one: http://ncptt.nps.gov...-documentation/ comparing photogrammetry to laser scanning.


There were other talks that were part of that event that would be useful too, but I don't see them up yet.  Not all the talks from the event seem to be tagged in a way that makes them easy to find.


I hope these resources are useful.



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#354 Summation quote of RTI process

Posted by davidturk on 04 March 2013 - 02:28 PM

I attended the RTI seminar in Indianapolis in September.  Our marketing dept. is writing an article on my experience, & there is a quote Marlin used in the introductory segment I'd like to use.  It was describing the RTI process as something like, "Simple concept, complicated procedure".  Unfortunately, I didn't write it down.  Does anyone have this quote?  tia.



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#317 Please support Cultural Heritage Imaging

Posted by Carla Schroer on 12 December 2012 - 11:52 PM

Dear Forum users,

Have you ever used Reflectance Transformation Imaging (RTI)? If so, you are a member of a select group. That group needs your help!

We love how the RTI community helps and supports each other on this growing forum. We know many people use the information here to solve problems they encounter, including those who find the answers they need without ever creating an account. We are proud of our work in fostering the development and dissemination of open source software and freely available user guides to enable the fantastic uses of RTI that are spreading around the globe.

Cultural Heritage Imaging is a small independent nonprofit organization, and we need support from individuals like you to help us keep our programs going. We are pleased to offer this user forum as a free service to people adopting RTI and related technologies. We were fortunate to have a bit of initial funding and extensive volunteer time to get it started, but there is no ongoing funding for projects like this at CHI.

CHI also supports new software development, updates to the open source software and user guides that help everyone in this field. We manage a source code repository and make source code available upon request, as well as gathering information from users about bugs and new feature requests for the tools.

Please help support this work by making a donation to CHI. For folks in the US this is a tax-deductible charitable contribution. Will you consider donating $10, $20, $50 or any amount that you can manage to help us continue this work? Wherever you are, you can donate securely via credit card on our Donate page.

Thank you for your consideration!

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#1619 New geodesic domes

Posted by Kirk Martinez on 13 February 2017 - 10:09 AM

Hi all,

we just completed over a year of development on a geodesic dome system and installed the first on in the University of York. We're still writing up the details but there are some images etc on our website. Headlines are:

  • 65 1300 lumen LEDs
  • relatively portable - as it unbolts into a box of struts and some aluminium poles
  • quite fast - about 65s per capture
  • takes different cameras as long as they have an external wired shutter release
  • doesn't need a laptop to drive it - as long as you can focus and set camera exposure its standalone



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#1218 New article about using RTI for daguerrotypes

Posted by Hembo on 28 April 2015 - 05:54 AM

Check out our article about using RTI for daguerrotype recording.

Also, we are about to start the project to document all (known) Estonian daguerrotypesusing RTI.


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