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After removal of the Canon IR cut filter, more red and infrared light will reach the CMOS imaging chip. Although this is great for astro-imaging and infrared photography use, the colors of normal daytime images will be off and shifted to red. Here are images before and after filter removal:
________Before IR Cut Filter Removal____________________After IR Cut Filter Removal___________
There are two ways to correct the problem of this color shift:
1. Use of Custom White Balance 2. Use of Color Correction Filter
Method 1. Use of Custom White Balance:
The Canon Digital Rebel has a custom white balance feature that allows you to take a picture of a white object, such as a sheet of paper or white T-shirt, that is then used by the camera to color correct future images. Here is a picture of a white sheet of paper taken by the camera after IR filter removal under sunlight with the camera in Auto White Balance (AWB )mode:
The above image was then used by the camera in its "Custom White Balance" (CWB) mode and a picture was taken of the same white sheet of paper:
The above image is now color correct for the ambient lighting (sunlight). If a picture is then taken indoors under (incandesent) room lighting with the same custom white balance setting, the color will be shifted again and not be correct. Here is a picture of a white sheet taken indoors with the "sunlight" CWB setting still in the camera:
To color correct the above indoor image which is now too yellow, the CWB feature would need to be used again by taking a picture of a white object under the indoor lighting. Here is a picture of a white sheet of paper taken with the AWB setting using indoor (incandesent) lighting:
The above image was then used by the camera in its "Custom White Balance" (CWB) mode and a picture was taken of the same white sheet of paper under indoor lighting for a corrected color version:
A disadvantage of using CWB is that the CWB white light image must be taken and CWB set every time lighting conditions change. That is one reason the AWB feature is so popular.
Here are the images before filter removal and after filter removal but using the Custom White Balance feature:
________Before IR Cut Filter Removal____________________After IR Cut Filter Removal with CWB________
The CWB image on the right above would need some tweeking of the color levels with software to get the color balance correct.
In addition to using the CWB feature for normal photography, an IR block filter should be used to keep infrared light from reaching the imaging chip. The above photo on the right was taken with a Baader UV/IR Cut Filter mounted to the camera lens.
Method 2. Use of Color Correction Filter:
Instead of manipulating the color balance after the increased red and infrared colors are captured by the imaging chip, as was done in Method 1 with AWB and CWB, another way to return the camera to its pre-filter removal state is to add a Color Correction Filter to the camera lens. The color correction filter should have the same spectral response curve as the original IR cut filter removed from the camera. The closest filter I could find is the "Color Correcting X-NiteCC1 (IR Blocking) Filter" by maxmax.com. Below is a comparison of the spectral response curves of both the X-NiteCC1 and original Canon filters:
The color of the original Canon IR cut filter and X-nite CC1 filter is very close by eye:
Testing I have done to date show that the X-nite CC1 filter is an ideal color correcting filter for the modified Canon Digital Rebel. Below are before and after Canon IR removal pictures using the X-nite CC1 filter on the same camera lens with no processing:
The X-nite CC1 filter I am using is 58mm in size and can be used with my other Canon lenses using step rings. The X-nite CC1 filter is available in a few different sizes.
Because the X-nite CC1 filter passes a little more ultraviolet than the original Canon filter, the addition of a UV filter may help under some lighting conditions.
Here is the camera with the X-nite CC1 filter installed:
Conclusion:
Although the custom white balance feature can be used to color correct daytime photos with some processing, use of the X-nite CC1 filter on the camera lens restores the color blance to the same as the pre-modification camera.
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Gary