Part XIV Overview: Build a Sensitometer

Part XIII produced a real curve with an honest apology attached. The exposures ran four to sixteen seconds under a tungsten lamp still warming as it worked; the wedge sat wherever it landed on the baseboard; the light across its 127 mm fell away towards the ends. Every one of those is a property of the apparatus rather than of the film, and every one of them is inside the answer.

A sensitometer takes them out. It is not a better enlarger. It is a box with one job: to put the same measured quantity of light on the same piece of film for the same length of time, today and in six months, so that when two curves differ you are entitled to say the difference is the film, the developer or the process.

The four things it controls, and the fifth the wedge provides

Exposure is illuminance multiplied by time, so the instrument must fix four quantities. How much light reaches the film plane. What kind of light it is, since a panchromatic emulsion and a lux meter weight wavelengths quite differently. Where the light goes — uniformly, because a gradient across the strip adds itself to the answer. And for how long: short enough to stay where the reciprocity law holds, repeatable far beyond the differences being looked for.

The wedge controls none of them. It supplies the ladder: twenty-one known fractions of one exposure in a single shot, so a whole characteristic curve comes off one strip developed in one tank at one temperature. Kodak’s own account of a sensitometer is exactly that short — a light, a shutter, a filter, a holder, a step tablet and the film — with illuminance times time read on a lux meter.

The design, in one paragraph

A single LED run at constant current, so that its output chases neither the supply nor its own junction temperature; an opal diffuser turning a point into a small uniform panel; a calibrated 21-step wedge in contact with the film; a microcontroller switching the LED for an interval timed on a crystal-referenced counter; and a light-tight box holding the same geometry every time. A monitor photodiode watches the lamp during the exposure and logs what it saw beside the timing, so a bad exposure can be identified instead of averaged in.

What you build and measure

Two builds: the light source and timer, bench-tested before it goes near a box, and the enclosure and exposure stage that gives it a diffuser chamber, a wedge holder and film registration. Then two experiments: calibration, which fixes the instrument’s scale and uncertainties and issues it a certificate, and process control, which sets it to pre-exposing control strips that ride every film run for weeks until the plotted result becomes a diagnosis. The measurements that matter are uniformity, illuminance at the film plane, repeatability over ten exposures, and drift from switch-on.

What it will honestly claim

Relative log exposure to a few hundredths, which is what comparing two developers needs. Absolute log exposure to nearer a tenth, because a consumer lux meter and a phosphor white LED do not make a photometric chain. No ISO speed at all: the course states its own criterion and marks every figure as its own measurement under it, as Part XIII settled. And a spectral mismatch with the standard sensitometric illuminants that no amount of care removes.

Against the alternative that is a good instrument. Hurter and Driffield’s unit was a standard candle at one metre for one second, and they reported densities of 0.750, 0.730 and 0.720 on three separate days. A box that beats that spread by an order of magnitude is doing real work.

What you need first

Part XIII complete, with its step-wedge negative and control strip archived, because the first thing this instrument does is disagree with them. A 21-step transmission wedge — the Stouffer T2115 is the course’s reference, and the calibrated CC parts earn their premium here in a way they did not before. Ordinary hand-tool competence. No electronics at all is assumed: the first page starts at charge and current.

⚠Everything here runs at USB voltage, and that is a decision

HSE’s guidance states that one of the best ways of reducing the risk of injury from electrical equipment is to limit the supply voltage to the lowest needed to do the job, and says plainly that 230 volts AC can kill. Every build in this course takes that route — a certified USB supply or power bank, five volts in and three and a third volts of logic — and nothing mains is built, modified or opened anywhere in Part XIV.

Mains appears nowhere in this course as something a reader builds, wires or switches. Part XVII’s timer was designed with a mains channel and lost it: a market survey on 5 September 2026 found no sealed, certified, low-voltage-triggered appliance switch on sale to a hobbyist, so the channel was removed and the instrument has one low-voltage output. The classification rubric puts home-built mains construction at Level C, which is a level this course does not ask anyone to work at.

What is present is a soldering iron, its fume and a bright emitter near your eyes. The rubric names low-voltage electronics builds and high-brightness LED sources at Level B, and the build pages are classified accordingly.

Three routes, none of them a consolation prize

Solder the module — the reference route, which the firmware and schematics assume. Breadboard it — every part in the bill of materials comes in a through-hole package, and a breadboarded instrument taped inside the box measures as well as a soldered one until something is knocked. Build nothing — Part XIII’s enlarger method still works, and this part’s design lesson says exactly what it can and cannot support rather than dismissing it.

The electronics are pocket money; the calibrated wedge is the one expensive item that cannot be improvised. Dated figures live in the laboratory planner, which does not yet carry lines for the electronics and says so.

The pages of this part

Part14 of 28Level3 — AdvancedPages6Estimated time13.3 hoursHighest safety levelLevel B

✱3 of 6 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 6 lessons in this part completed

• LessonLow-Voltage Electronics for the Darkroom Builder75 min
• LessonSensitometer Design Principles60 min
• BuildBuild: The LED Light Source and Exposure TimerB180 min
• BuildBuild: The Enclosure and Exposure StageA✱ Darkroom180 min
• ExperimentExperiment: Calibrating the SensitometerA✱ Darkroom · Mains150 min
• ExperimentExperiment: Process Control with Control StripsA✱ Darkroom150 min