A Brief Introduction

article by Ben Lincoln

This article is intended as a quick-and-dirty high-level overview of multispectral photography. It should give you the bare minimum necessary to understand what's being represented on this website. A lot of information is simplified, and there is much glossing-over of details. With that having been said...

Human eyes are sensitive to three primary colours: red, green, and blue. Modern technology allows us to obtain images of parts of the spectrum outside this limited area, but it can't (yet) give our eyes and brains new primary colours to represent them. This results in two main options: view this other imagery as discrete greyscale ("black-and-white") pictures, or create "false colour" images that use the familiar three primary colours to represent other bands of the spectrum. For example, instead of "red-green-blue", a photo may represent "green-blue-ultraviolet" or "infrared-red-green" (by mapping those three bands to the red, green, and blue that our eyes can actually see. The corresponding notation on this site would be "G-B-UVA" and "NIR-R-G" respectively, compared to the traditional "R-G-B".

Here's a comparison of the spectrum generated from a prism, as interpreted using some of these variations:

Spectrum False Colour Comparison

False Colour Comparison-Stripe

The same slice of the prism-generated rainbow as represented using a variety of false colour methods. The 1000nm-880nm-720nm stripe is a "miniature rainbow" that exists entirely in the near infrared.

Date Shot: 2009-10-04
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 50mm
Filters: LDP CCI, LDP 1KB, B&W 093, Hoya R72, Baader U-Filter
Date Processed: 2009-10-04
Version: 1.0

This is how those variations play out in a real-world image of a flower bed:

Petunias 3

R-G-B

NIR-R-G

NIR-R-B

NIR-R-UVA

NIR-G-B

NIR-G-UVA

NIR-B-UVA

R-G-UVA

R-B-UVA

G-B-UVA

NIR-RGB-UVA

The petunias as represented using the same variety of false colour methods.

Date Shot: 2007-06-17
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 28mm
Filters: LDP CCI, B&W 093, B&w 403 and LDP CCI stacked
Date Processed: 2009-09-14
Version: 2.0

False colour is a powerful tool for representing information - note especially how the ultraviolet patterns of the petunias (which are completedly invisible to the unaided human eye) become visible in the variations which include that spectral band.

When you see a variation such as "Tinted NIR", this indicates that the image is greyscale but has had a solid colour "wash" applied to (hopefully) make it more visually pleasing. This is the equivalent of early photographs being sepia-toned - it represents how the photo was created and/or processed, not that there is anything sepia-coloured about people from the 19th century. Similarly, there is nothing "red" about infrared, or "violet" about ultraviolet. Their names simply mean "below red" and "above violet". You could call red "below green", but that wouldn't make it green itself.

There are some additional variations I will usually produce of each photo, but for purposes of this short introduction you can consider them "pretty pictures that don't represent anything useful", which isn't too far from the truth.

If you are at all interested in this topic, I highly suggest proceeding on to A Detailed Introduction.

Footnotes

1.	"NIR" being an abbreviation of "near infrared", and "UVA" being an abbreviation of "ultraviolet-A". The distinction between various types of infrared and/or ultraviolet is one of the details that this extremely short introduction omits.