Saturday, March 22, 2008

Making a Backlight

As I said before, the LCD screen on my MT table does not let off its own light. It needs to be backlit for the images to be seen. To date, I haven't found a very good way to backlight my table, but here is what I have tried so far.

For my first attempt, I tried to use the original backlight of the LCD monitor. The backlight from the monitor included two light sources, one on the left and one on the right, each on containing two extremely thin fluorescent bulbs. These two light sources shined into the sides of a piece of glass about 3/8" thick. This glass was frosted with a grid of ridges. I believe the backlight actually used FTIR to get a nice even light across the screen. As the light was shined inside the glass, the ridges on the back of the glass would disrupt it so the light came out of the glass evenly across the entire frame.

I hooked these lights and glass up to my table under the LCD panel using some mirror clips and plenty of duct tape. While this created a good backlight, it had a few problems. The first problem is that the frosted glass blurred my infrared blobs quite a bit. Also, having the light source above the camera introduced a lot of infrared light that the camera could see, which further obscured my blobs. The final problem was that the fluorescent bulbs were extremely thin, and many of them broke during my assembly. In the end, I removed this backlight setup.

My second try at a backlight was just a simple lightbulb installed at the bottom of the table, near my camera. This didn't look nearly as good as my first approach. I found the light from the light bulb was more yellow, causing discoloration of the image. The light was also not as bright, and less evenly distributed. The blob tracking was also adversely affected by this backlight. The lightbulb flooded the area with too much infrared light, even though I had used a fluorescent bulb, which gives off less heat and infrared than incandescent bulbs. This drowned out the blobs of light of my finger touches, and illuminated parts of the screen that caused extraneous blob detection.

My third try at a backlight is my current revision. I bought two under counter fluorescent bulbs and attached them at either side of the LCD panel, under the tabletop. This caused brighter illumination at the sides of the screen, because there was no diffuser like in the first backlight setup. I tried to fix this by attaching tin-foil reflectors to direct the light, but they didn't have a noticeable affect. The illumination is sufficient though. These bulbs also let off extra IR light, but by moving the camera closer to the LCD panel, so it doesn't pick up the light bulbs, I was able to negate a lot of this effect. I also covered anything that would reflect the light bulb's infrared light with black electrical tape, so they would not reflect light.
While my current backlight serves its functions, I believe that there are serious improvements to be made. I will continue working on that...


Blogger Carey and Philippe said...

Have you thought of using the old school transparent projectors. They can be bought for pennies on ebay. Or borrowed from school!

May 2, 2008 11:58 AM  
Blogger Joshua said...

have a look around at photography websites for a 'soft=box' then have a look at using circular fluro lights for back illumination, that way you can mount your IR camera 'inside' the light or even above it!

May 4, 2008 11:06 PM  
Blogger Sirius said...

maybe if you put several mirrors into the base to distribute the light of a hidden lamp in order to the light arriving in the LCD is fulling distributed around on the surface

October 5, 2008 7:44 PM  
Blogger Sirius said...

PS: like a Kaleidoscope

October 5, 2008 7:45 PM  

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