Densitometer: ambient leak

Lifted from test T3 on the densitometer optical head and the stray-light section of photodiodes, ADCs and measuring light.

What you see

A reading that depends on the room. The same sample, undisturbed, reads one number with the desk lamp on and another with it off. In the worst cases somebody walking past the bench changes the third decimal.

Part XV says why this one deserves an entry of its own rather than being folded into stray light: a fault that only appears when the desk lamp is on is one you would otherwise diagnose as drift for weeks. It looks exactly like an unstable instrument until the moment somebody thinks to try the light switch.

Likely causes

1. The shroud, or its seal. Foam that has compressed, a lip that no longer meets the stage, a hinge that holds the shroud a millimetre proud.
2. Box seams around the sample stage that were never light-tight, or that have opened.
3. A sample too large for the shroud, holding the seal open along one edge — a sheet film or a print will do this where a wedge does not.
4. Infrared. Silicon’s peak sensitivity is near 920 nm; a tungsten desk lamp or a heater is a strong emitter there, and neither is visible as a leak to the eye.

What is happening: the chemistry and physics

Ambient light adds to the detector’s signal by the same arithmetic as stray light — a fixed quantity added to every reading, compressing the dense end and leaving the clear end alone — but it comes from outside the instrument rather than from the instrument’s own lamp, and that is the whole distinction.

The consequence is that the two faults have opposite cures. Blackening the interior does nothing for ambient leakage, because the light is not bouncing around inside; it is arriving. Sealing the shroud does nothing for stray light, because the light never left the box.

The infrared half is the reason the leak is so hard to find by eye. The detector is up to three times more sensitive at 920 nm than at a green measuring wavelength, so a leak that admits light you cannot see admits signal the instrument certainly can.

For scale: a professional instrument in a sealed metal case specifies its ambient interference as a decrease in D of less than 0.25 per cent — quoted as a percentage of the reading rather than of full scale, and a figure a plywood head should expect to lose badly against on a first attempt.

!The confirming test: T2, twice, with the lights

Run the stray-light test twice — once with the room lights on, once with them off — and compare.

• The two agree: whatever compression you have is your own lamp, and the cure is blackening and baffling.
• The two differ: that difference is the ambient leak, and it is quantified in the same units as everything else on the certificate.

Then find it: put the instrument’s lamp inside the closed head in a dark room and look, and separately shine a torch at the closed head from outside while watching the reading. The first finds the paths light takes out; the second finds the paths it takes in, and they are not always the same paths.

Diagnostic questions

• Does the reading change with the room lights? Ten seconds, one switch, and the diagnosis is made or eliminated.
• Does it change with the sample in place? A leak that only appears with a large sheet loaded is the shroud being held open by the sample.
• Is there a heater, a tungsten lamp or a window in line of sight? All three are infrared sources and only one of them looks like one.
• Has the foam aged? Compressed foam is a seal that used to work, which is worse than one that never did.
• Is the fault repeatable? Ambient leakage is: the same lights give the same offset. Genuine drift is not.

Corrective action

Fix the seal and the seams, not the paint. Part XV’s troubleshooting table is explicit that blackening the interior will not help this fault.

Re-run T3 to confirm, and record the residual difference on the certificate as the instrument’s own ambient-interference figure.

Until it is fixed, work in a dark room and say so on the readings. A measurement taken under stated conditions is usable; one taken under unknown conditions is not.

Prevention

Make the shroud oversize and compliant, so that a sheet of film does not hold it open.

Include T3 in the routine check alongside the dark reading and the stray-light floor, because seals age and seams open.

Keep the instrument away from tungsten and from heaters, on the same grounds that keep it away from the radiator in the drifting-zero entry.