stray light
/ STRAY lyte /
Imagine trying to hear one quiet voice while a faint hum leaks in from the next room. No matter how dark you make your sample, a little of that unwanted hum always reaches the meter. In a spectrophotometer that unwanted leak is stray light.
Stray light is any radiation reaching the detector that is not the wavelength the monochromator is supposed to deliver — light scattered off optics, leaking around the slit, or of the wrong colour entirely. Because it slips through even when the sample blocks the intended beam, it puts a floor under how dark a reading can ever appear.
Stray light is the main reason Beer-Lambert law fails at high absorbance. When the true transmittance is tiny, the leaked light dominates, so the instrument under-reads the absorbance and the calibration line bends flat. Good instruments minimise it, and analysts avoid the high-absorbance region where its effect is worst.
If 0.1% of the beam is stray light, a perfectly black sample can never read above an absorbance of 3, because that leaked one-thousandth always gets through to the detector.
Stray light caps the highest absorbance an instrument can truthfully measure.
Stray light errors always push absorbance readings too low, never too high, and they bite hardest exactly where samples are most concentrated.