Monday, September 22, 2014

DSLR Full Spectrum Conversion

Color infrared image captured with camera modified in this post. Boring composition, but just look at those colors...

I have converted several digital cameras to full spectrum and I figured I would share my experiences. So far I have only experimented with near infrared (NIR), but the conversion should allow one to use the camera for ultraviolet (UV) and natural color photography as well.

If you are wondering why infrared photography is so cool, just look up "Aerochrome". Aerochrome was an aerial photography film sold by Kodak that has unfortunately been discontinued. Infrared photographs of crops and forests can be used to analyze the health of vegetation. Infrared photos can help distinguish camouflage from real vegetation. The film also made for some really surreal photos on the ground. Note that Aerochrome produced color infrared images. Black and white infrared films also existed, and a camera modified as described can produce these as well. In these vegetation will be white rather than red.


Can you spot the vegetation?

See how easy it is to distinguish the living plants from the dead ones?


This post is going to describe the camera conversion, in a later post I will describe how to use it, and the image processing required to produce a color infrared (CIR) image that emulates Aerochrome.

The sensor in a digital camera is actually sensitive to wavelengths of light outside of the visible spectrum (red to violet). This includes infrared and ultraviolet light, wavelengths below and above visible light. Cameras are sold with a filter, also known as a hot mirror, that blocks infrared light from reaching the sensor. This prevents the infrared light from contaminating the image. By replacing this filter with clear glass, we can photograph infrared light. The filter we remove is actually a stack of filters, laminated together, and also contains the anti aliasing filters. The camera will work without them.


Hot mirror and anti-aliasing filter stack is in the black frame attached to the sensor


Note that I am talking about near infrared. This means the wavelength is around 700nm to 1100nm or so. It is still light reflected by an object. Thermal imaging, which people typically think of when they hear "infrared imaging" is actually longwave infrared (8000nm to 25000nm) and requires a different type of sensor all together. These modified cameras cannot do thermal imaging. They can, however, see infrared emitted from very hot objects, such as this soldering iron (~650F). An electric stove will also glow in a NIR image before the eye can see it, experiment!


Normal color photo of hot soldering iron

NIR photo of hot soldering iron. #12 filter, ISO 100, f/2.8, 0.5" exposure directly from camera modified in this article, no post processing


Before we go any further let me say that there is a good chance you will ruin your camera when doing this conversion. I have done several, and I have ruined a couple cameras. Don't do this on your only camera, don't do this on a camera you can't afford to replace. Wear a ground strap to prevent static damage to the electronics. Do this at your own risk.  

The flash capacitor does not discharge itself when you remove the battery. It contains enough energy to melt the tip of a screwdriver. At the very least you will receive a painful electric shock if you put your fingers in the wrong place. If you don't understand this risk do not take apart any camera with a flash. You have been warned.

Here is the Canon 20D that will be converted to full spectrum

I am not going to give a step by step guide to disassembling the camera. These are all over the internet. Instead I will provide some notes that I find useful.

Ribbon connector

To remove most ribbon cables, flip the little black latch up with a small tool. Don't pull hard on any cables to disconnect them, if it won't come out easily you don't have the connector unlatched correctly.

Back of camera showing noise shields

Do not be tempted to cut the copper foil that is holding some parts together. These are noise shields and need to be intact after reassembly. You can peel them off like you would with a sticker. Similarly, desolder any noise shields that are soldered in place and in your way. Make sure you solder them back in place when you reassemble.

Filter stack removed from sensor


Once the sensor is removed, you can take the filter stack and it's holder off of the sensor. Put the sensor somewhere it won't get dusty.

Filter stack gasket and retainer removed

Remove the gasket carefully so as not to tear it. I use a small knife. The metal retainer clips on in four places, remove it too.

Hot mirror removed from holder

Cut the adhesive that holds the filter stack in the holder. You will want to remove as much of the old adhesive from the holder as possible.

You can't just remove the hot mirror and anti-aliasing filter stack and put the camera back together and call it good. You need something to take the filter's place, both to keep dust off the sensor and to keep the focal point in the right place. Changing the thickness of this filter, or the type of glass (we are doing both) is going to shift the focal point due to refraction. Think about what happens when you put a pencil halfway in a glass of water... This filter is not on the light path to the autofocus sensor so the autofocus will not correct for this. It is not on the light path to the viewfinder so you cannot correct for this by focusing manually. If your camera has live view, focusing using live view will allow you to correct for this. This shift can be calculated and is likely fractions of a millimeter. In my experience, every SLR I have converted has focused perfectly if I replace the hot mirror/ low pass filter stack with a cut up lens filter. Maybe I am just lucky. I have used both clear protection filters and skylight filters. (Don't use a skylight filter if you want to try UV photography.) Your results may be different, if you try this and have issues, try calculating the focal point shift and shimming your sensor. Instructions for this can be found online, or in a physics book. The shims are those small copper plates that sit between the sensor's mounting bracket and the mounting bosses on the camera. Remove them to move the sensor forward, and add more to move the sensor back. 

Tracing the old filter on the new filter

I used a plain glass protection filter to make my replacement filter for this camera. I cover the filter with packing tape and trace the old filter.

Fiber reinforced cutting wheel (back) and diamond cutting wheel (front)

Use a fiber reinforced cutting wheel to cut the metal ring of your filter so the glass can be removed.

Glass removed from metal ring

Now use the diamond cutting wheel to cut the glass. Go slowly to prevent heat build up. It doesn't take much pressure. Don't forget eye and respiratory protection, it is not good to breathe glass dust.

New filter set in holder

Once the filter is cut out, trim and adjust it so it fits snugly in the holder. Don't worry if you end up with some small chips around the edges, they will be covered by the mask. Remove the packing tape. Clean the filter, removing any tape residue, glass dust, and fingerprints. I use soap and water in this step.

Make sure the filter is sitting straight in the holder, and is flush with the outside. This is the side away from the chamfer, or down in the above picture.

Adhesive applied

There is a chamfer on the inside of the filter holder. With the new filter in place, fill this chamfer with black silicone adhesive. It doesn't take much. If you smear a little on the glass don't worry, we will clean it when it cures. Don't try wiping it around when it is wet, as this will only make a bigger mess.

Do scrape any high spots down with a straight edge, holding it angled toward the outside edges of the plastic filter holder, and avoiding contact with the glass other than the edges.

Removing excess silicone

My favorite way to remove excess silicone is with a pencil eraser. Once it has cured, lightly rub the adhesive off of the glass.

Cleaning tool

Now is the time to clean this glass really well. I use PEC pads and methanol. Isopropyl alcohol will not work for this, it doesn't evaporate fast enough. If you fold up the PEC pad like shown, you can use a bread bag tab to apply even pressure as you wipe the glass. Make sure you get it nice and clean, any dust or lint or streaks on this glass will show up in your pictures. The needs to be much cleaner than a lens does. Think of it like a negative, any little bit of dust shows up clear as day when scanned or printed. Spend a good amount of time cleaning this.

Ready for installation

Reinstall the gasket and metal frame.

Ready for installation

Attach the new filter to the sensor and reinstall. Carefully put the camera back together, and make sure everything works. Don't forget those noise shields. If something doesn't work, recheck that all the ribbon cables are seated properly.

Ready to go

It doesn't look any different from the outside, so if you have two of the same cameras, mark it somehow.

I use a Yellow 12 filter for color infrared photography.  I have also had good results with a 767 spotlight gel simply held in front of the lens on point and shoot cameras. Typically I underexpose by 2 stops and be sure to set a custom white balance.

For black and white infrared you will need an opaque filter, such as a #89B or R72. The first camera I converted for black and white infrared was a tiny 1.3MP point and shoot. I cut a couple small squares out of the leader of an exposed and developed roll of color film. I put these squares between the lens and sensor. It worked great. A 789 spotlight gel also works pretty well, and the magnetic part of a floppy disk is said to work, although I have not tried that one. To use the opaque filters without live view, compose and focus without the filter, then install the filter. Rotate the focus ring so the number that is on the distance index is now on the IR focus mark. Obviously the light meter is not going to work, so you will have to play around with exposure time.

UV photography is frequently done with an 18A filter, although there are issues with this because an 18A filter passes a lot of NIR as well. This is not something I have experimented with, yet.

If you want to use the camera for natural color again you will need a "hot mirror" filter.

My next post will describe how to use this camera for CIR and why it works.



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