Anaglyph glasses transmission spectra
Testing the quality of anaglyph glasses by measuring the transmission spectrum and the optical denstiy spectrum.
Image projection and perception
As you probably know, anaglyphs work by adding together two images which are colored in complementary colors (like red and cyan, respectively), one for the left eye and one for the right eye. In order to separate the two cannels again, special colored filters are put directly in front of the eyes (usually the red one for the left eye and the cyan one for the right eye).
Obviously, this works perfect if both channels are projected using separate (non-overlapping) spectra (i.e. light wavelength ranges) and if both filters completely block one of the channels (i.e. the cyan glass should block the red channel and the red glass the cyan channel).
However, as we will see below, in real live, conditions are far from perfect leading to distortions.
There are two main types of perceptual distortions:
Fighting distortions: There is not much one can do against image
distortions. The best thing is to get glasses which work well for the
intended projection device.
Transmission and optical density
The quality of a filter can be described by looking at how much light can pass through the filter at all the different wave lengths of interest.
When talking about filters, there are two main properties of interest
are transmission (T) and optical density (OD).
They both describe essentially the same and the relation is:
The reason for having both of them is that a transmission graph, while showing nicely the amount of light transmitted at those wavelengths where the filter is (nearly) transparent, will not give away much detail at those parts of the spectrum where the light is absorbed. To put it in other words: In a linear transmission graph, you can very well distinguish 95% transmission from 80% but not 1% suppression from 0.5% or 0.1%. So, the optical density is a logarighmic suppression figure, OD 0 means full transmission, OD 1 means 10% transmission, OD 2 and 3 mean 1% and 0.1%, respectively.
Note: Since human perception has approximately a logarighmic characteristic, percepted brightness scales more like optical density rather than linear transmission percentage.
The glasses were measured using a Varian Cary 3 (UV to visible) Spectrophotometer. The device was allowed to heat up for some time and baseline correction scans for both 0% and 100% transmission were performed prior to measuring the glasses.
The spectrometer can measure up to OD 5, this is why optical density graphs look jerky above 5. However, since OD 5 translates into 0.001% transmission, anything above OD 5 isn't interesting anyway (think of it as "no light at all").
Note that the best glasses won't give you a good image unless the projection device is able to deliver a good image! Screens, for example, normally have overlapping spectra for different color channels (by design) leading to ghost shadows as pointed out above.