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An alternative for controlling 3W leds

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Hi,

probably old news for the experienced Dome users but I've just found a link to a 3W LED mounted on a pcb which only requires power and one control pin.

Rather than using MOSFETs etc etc a dome could be built using only one LED driver board with all the LEDS wired in parallel and a single control wire for each LED

https://www.ebay.co.uk/itm/3W-High-Power-KEYES-LED-Module-With-PCB-Chassis-For-Arduino-STM32-AVR-UK/232870808490?ssPageName=STRK%3AMEBIDX%3AIT&var=532402693460&_trksid=p2057872.m2749.l2649

I've ordered a handful to have a play with

Kev

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The Keyes 3W modules seem to be very optimistic on power rating. They only pull 0.17A at 5.7V so loosely only 1W. Using a Colorchecker calibration and setting a white balance on a grey square they are also running at 8200k.

I also have a box full of Bridgelux 6500k Daylight White 3W star pcb mounted LEDs. The LED board on the KEYES device is only two solder leads onto a star pcb and very simple to remove and replace with a Bridgelux module. The Bridgelux LCD is pulling 0.4A at 5.7v so 2.3W and is approximately one stop brighter and runs with a colour temperature of 6750K.

I suspect the 5.7V value is probably a bit low as I'm using an commercial LED driver to power the LEDS and the voltage waveform is likely to have some ripple on it.

The dome build will continue with the Bridgelux 6500K leds :)

 

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On 9/14/2018 at 3:27 AM, leszekp said:

You might give this site a look (not that I'm biased or anything):

https://hackaday.io/project/11951-affordable-reflectance-transformation-imaging-dome

Your site gave me the inspiration to start my own little project :)

Because my dome will only get used infrequently I've changed the fully automated system you've built to ease my build time. I'm going down the route of a dome with only seven 3W leds inside on a radial line from the bottom to the top of the dome. An arduino will control and synchronise the LEDs and triggering of a Canon 5D4 camera.

The duration of the LEDs will be defined by the camera shutter by sending a signal from the camera back to the arduino so it turns the LED off when the shutter closes and the sequence moves on to the next LED.

Once the first set of seven images are complete I'll manually rotate the dome 'x' degrees and shoot the next sequence. The dome is going to be mounted on a separate base so it is completely separated from the subject and the camera.

Kev

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I'm just getting into the programming of the dome LEDs and one question that keep niggling in the back of my head.

The LEDs at 65 degrees will clearly illuminate the subject more strongly than the LEDs down at 15 degrees, so the maximum camera exposure is defined by the  65 degree LEDs to avoid clipping the white levels.

This compromises the optimum exposure when images are being taken with the 15 degree LEDs as the subject will generally be underexposed with the shadows being clipped.

Is there any benefit in increasing the LED light output of the lower LEDs, which can easily be done in the software, to improve the exposures OR would this affect the RTI builder processing of the images because it might be expecting a constant camera exposure?

regards

Kev Lewis

 

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I had the same thought a few years ago, but decided that it didn't make sense. You're fitting a curve to the (primarily) Lambertian lighting scattering at every point based on the lighting angle. If you change the lighting intensity with angle, you'll skew the lighting curve to fit to those higher intensities, which will introduce errors.

Shadows are areas where the main light source is blocked, so any signal from those areas is a result of scattered light within the dome, which doesn't really offer you any useful signal for curve fitting.

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I'm with leszekp on this.  The software expects that nothing changes except the light positions.  The images from the lower angles should be darker.  Don't compensate with the lights, and don't exposure comp those images.

You might want to read through the guide to highlight image capture on the CHI website - even though you are building a dome - because some core principles like this are described there.

Carla

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Thank you both for your input :) I did read through the various articles on capture and processing but the idea of modifying the lighting levels was just niggling away in my head so I asked the question :)  I have programmed the LEDs so they are outputting the same amount of light, the only correction I've made is to calibrate all of the LEDs output so they are identical within 1-2%. The errors are just from tolerances of the components and the LEDs themselves, the difference between the 'worse' and 'best' LED was less than 5% but the correction was a simple fix.

best regards

Kev

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MY dome build update: the control system is complete and programmed and the dome is marked out and sprayed matt black. I bonded one of the 3W leds on a star pcb to a piece of the acrylic the dome was made of and ran it with an IR non contact thermometer to check what temperature they fail at before installing the LEDs into the dome. They get HOT lol after approx 3 minutes the adhesive failed (turned to liquid) so I've ordered some high temperature silicon adhesive to use on the dome, which is rated to 300C which will be more than adequate. FYI I won't ever let the LEDs get to maximum temperature, the software will stop that, but it's nice to know what happens in advance of making a mistake.

Kev

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Running the 3W LEDs continuously is definitely a bad idea when there's no heat sink. The solder melting point is less than 200C. They only need to be on long enough to take your photo, which shouldn't be more than a few seconds. While many of these are rated up to 1A, the usual nominal operating current is 700mA, at which current level they will stay quite a bit cooler. Intensity is a bit lower, and the exposure time will be slightly longer, but it's not going to be a huge different (a few tenths of a second).

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On 10/6/2018 at 12:39 AM, leszekp said:

Running the 3W LEDs continuously is definitely a bad idea when there's no heat sink. The solder melting point is less than 200C. They only need to be on long enough to take your photo, which shouldn't be more than a few seconds. While many of these are rated up to 1A, the usual nominal operating current is 700mA, at which current level they will stay quite a bit cooler. Intensity is a bit lower, and the exposure time will be slightly longer, but it's not going to be a huge different (a few tenths of a second).

I've just finished the build and run my first test :) VERY pleased even if I did miss one row of the sequence because of a camera buffering problem that I didn't notice quickly enough. The LEDs are powered via 700mA PWM controllers so I can play with the light output and typically they will never be on longer than a few seconds (the longest time, at a lower output, will be while I'm focusing and adjusting stuff before starting a run).

This is my first attempt with a RTI dome all the images were shot with a Canon 5D4 and Canon 100mm macro lens 1 second exposure at f/16 iso 200 .

For info the camera buffer was because I was being lazy and had the camera connected to my laptop via wifi rather than on a usb3 cable so the camera missed a few triggers because it was 'busy' transferring 30Mb raw files 

thanks for all your input leszekp it's been invaluable

Kev

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I had noticed that it was somewhat conductive, probably because it's full of graphite. But didn't see any effects in my system.

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