Each optical stage multiplies the signal by a transfer fraction, so the final detector output is the source times every stage ratio in sequence. The source signal 24 passes through a lens stage of 3/4 giving 18, then a filter stage of 2/3 giving final output 12, so 24 times 3/4 times 2/3 = 12 exactly for the stated toy transfer ledger; real optical chains have alignment loss, wavelength dependence, detector response, stray light, and measurement uncertainty.
An optical chain can be checked as exact transfer fractions. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The source starts the optical budget
The source signal is 24 training units before any sample claim.
Ssource=24
Stage transfers multiply
The lens transfer is 3/4 and the filter transfer is 2/3, so the output is 12.