Wavelengths are stored in the ctype field and determine the colour of the elements (e.g. PHOT, FILT, BRAY, BIZR, BIZS and BIZG.) They can be used as storage by setting the wavelengths to a certain amount and read that info by filtering out some wavelengths and letting others through. https://powdertoy.co.uk/Browse/View.html?ID=2332971 shows that use. Wavelengths are displayed at the HUD on top of the element name. In a full spectrum, the wavelengths on the left are red, and on the right they are blue. In between are the colours of the rainbow. When it isn't a full spectrum, then some colours will be missing. To set a wavelength, you need to set the ctype to get different wavelength. The wavelengths are: Wave NO . Ctype

 Blue wavelengths Rightmost
 1.1
 2.2
 3.4
 4.8
 5.16
 6.32
 7.64
 8.128
 9.256
 10.512
 11.1024
 12.2048
 13.4096
 14.8192
 15.16384
 16.32768
 17.65536
 18.131072
 19.262144
 20.524288
 21.1048576
 22.2097152
 23.4194304
 24.8388608
 25.16777216
 26.33554432
 27.67108864
 28.134217728
 29.268435456
 30.536870912
 Red Wavelengths Leftmost

You can add them together, e.g. to get both wavelengths, 30 (the leftmost and the reddest wavelength) and 1 (the rightmost and the bluest wavelength) in the same spectra, you need to add the corresponding ctypes in the chart. 30.536870912 ctype for wave 30 = 536870912 1.1 ctype for wave 1 = 1 536870912 + 1 = 536870913, so set the ctype of the PHOT or FILT or whatever to 536870913 to get both wavelengths 30 and 1.