Daytime Infrared Hoya R72 Filter:
A popular filter for infrared imaging is the "Hoya R72". When I modified the Canon Rebel 450D, I replaced its Canon IR Cut filter with a Baader "DSLR Correcting Filter" which is a much milder UV-IR-Cut filter that allows the H-Alpha wavelength to pass for deep sky. The CMOS imaging chip is receptive to all wavelengths of light. The visible light range is from 400 nm (violet) to 700 nm (red). Wavelengths above 700 nm and up to about 5000 nm are known as near infrared. Here are the transmittance curves showing the original Canon IR Cut filter (that has been removed), the Baader UV-IR filter (that has been installed in the camera body), the X-Nite CC1 color correcting filter that can be added to a lens for color correction and the Hoya R72 infrared filter that can be added to a lens for daytime infrared imaging:
Based on the published curves of the filters above, the Baader UV-IR filter blocks wavelengths above 700nm and the HOYA R72 infrared filter blocks wavelengths below 700nm, so with both filters in the optical train, no light (or very little) should be reaching the 450D's imaging chip. When I first tested the modified 450D with the HOYA R72 filter I was surprised to see the amount of infrared light that was captured and this is still puzzling to me.
One thought is that the 450D's CMOS imaging chip is sensitive to wavelengths greater than 1100nm where the Baader UV-IR cut filter begins to pass light again as can be seen on the following transittance curve published for the filter by Baader?
Another reason might be that the published filter curves are not very accurate. On Christian Buil's Astronomical Filters Spectral Transmission web page, he has tested the Baader UV-IR filter and has a measured transmission curve HERE. The curve shows that there is some minor level of infrared light being passed just above 700nm (7000A), as per the area shaded in yellow below:
In the photos below, an X-Nite CC1 color correcting filter was used for the normal image and the Hoya R72 filter was used in its place for the infrared image. Focusing through the camera's viewfinder when using the R72 filter is not possible because it is a very dark red, nearly black, filter. Fortunately, the 450D has a "live view focusing" feature by which the camera displays the actual image being recorded on the imaging chip. For outdoor daylight imaging in moderate light with the HOYA R72, the "live view focusing" camera display gives an easily seen infrared image for focusing, even using the 5X and 10X modes.
Normal versus Hoya R72:
These are RAW images converted to JPGs and resized with no further processing:
The following image is a "mouseover image". You will need to have scripting allowing in your web browser for this to work. Move your mouse cursor over the normal image using an X-Nite CC1 color correcting filter and you will see the infrared image of the same scene using the Hoya R72 filter. Move the mouse cursor off the image to go back to the normal image:
Move cursor on and off image:
In the above photo, imaging in infrared resulted in vegetation such as plants, shrubs, trees and grass appearing bright (white). Canera settings were:
Normal: ISO 200, F10, 1/200 second, 55mm, AWB and Auto focus.
Infrared: ISO 400, F5.6, 1/8 second, 55mm, AWB and Auto focus.
The following image is a "mouseover image". You will need to have scripting allowing in your web browser for this to work. Move your mouse cursor over the image and you will see the infrared image of the same scene. Move the mouse cursor off the image to go back to the normal image:
In the above photo, imaging in infrared resulted in tree leaves and grass glowing very brightly. Camera settings with kit lens were:
Normal: ISO 200, F9.0, 1/200 second, 18mm, AWB and Manual focus using live-view.
Infrared: ISO 400, F4.5, 1/10 second, 18mm, AWB and Manual focus using live-view.
UPDATE:
Based on the results above, I suspected that the DSLR version of the Baader UV-IR filter may not have the same filtering characteristices of the versions sold in 1.25" and 2" cells for astrophotography. So I held a "Baader IR Blocking Filter 2" (optically polished APS-coated #2459210)" in front of the HOYA R72 infrared filter and it blocked out more infrared light as shown below. I don't know if stacking two IR cut filters would decrease the infrared light to this degree; or maybe the DSLR version of the filter is just weaker than published curves.
Camera Settings: ISO 400, F5.6, 1/10 second, 18mm, AWB and Manual focus using live-view.
To learn more about Infrared Imaging with a Digital Camera, see the Digital Photography For What It's Worth article by Jeremy McCreary: Infrared (IR) Basics for Digital Photographers—Capturing the Unseen