Red-Free Imaging

Tue, 4 Jun 2013 15:02:09 GMT

To the gurus of nanometers, I have a basic question.

Does imaging using the green red-free filter and then turning it to monochrome give you the same results as capturing the image in color, opening it in Photoshop and looking only at the green channel?

-Leslie

Tue, 4 Jun 2013 16:42:08 GMT

In reality, not exactly, but for practical purposes it may not make a big difference. The green channel of a full color image probably covers a braoder wavelength band that some filters used for "red-free" imaging. If you were using a very wide-band green filter (like a wratten 58, which is the color separation filter meant to reproduce the green "channel" on film) it would be similar monochromatic information as the green channel in a digital file.

The red-free filters used in fundus cameras are not all the same in spectral trasmission. Some lean a little yellow and most probably have a narrower bandwidth than the wratten absorption filter. So in this case it wouldn't be quite the same green info.

Another factor is whether you are using a color sensor (set to grayscale) vs. a monochrome sensor with a different spectral sensitivity range. The color sensor uses a Bayer filter (a micro-mosaic of RGB mini-filters) to distribute the color info across all three RGB channels. When you use a green filter, you are essentially only useing the green sensitive pixels on the sensor during capture. That puts you pretty close to the same end result. The biggest difference between the green channel image vs. red-free filter image occurs with monochrome sensors.

Attached is a slide from my wavelengths talk which shows 4 different green images. They all look slightly different, but unless they were side-by-side, I'm not sure you coud tell how they were obtained. Hipe this helps.