Photometer
Instrument for measuring light quantities like luminous flux.
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NASA Johnson Space Center · Public domain
A photometer measures photometric quantities like luminous flux, illuminance, or luminance. Before electronic sensors, photometry relied on visual estimation: a source’s luminous flux was compared to a standard.
The human eye judged when illuminance from both sources was equal, and the inverse-square law then gave the relative fluxes. A simple example used an oil spot on paper—when the spot vanished from both sides, the illuminances matched. By 1861, three common types existed: Rumford’s photometer, Ritchie’s photometer, and the extinction-of-shadows method, considered the most precise.
Rumford's photometer
Rumford’s photometer (or shadow photometer) worked on the principle that a brighter light casts a deeper shadow. Two lights cast shadows onto paper; if the shadows matched in depth, the difference in their distances indicated the intensity difference (e.g., a light twice as far was four times as intense). Ritchie’s photometer relied on equal illumination of surfaces. It used a box with a wedge of wood angled upward and covered with white paper.
Ritchie's photometer
The user looked through a tube at the top, seeing both surfaces illuminated by lights placed at the box’s sides. Adjusting the lights until both surfaces appeared equally lit allowed intensity comparison via the square of distance differences. The extinction-of-shadows method depended on a second light being brought to a distance where it completely erased the shadow cast by the first light on a white screen.
Photon counting
Most modern photometers convert light into an electric current using a photoresistor, photodiode, or photomultiplier. They may analyze light after it passes through a filter or monochromator for specific wavelengths or spectral distribution. Some photometers use photon counting, measuring light by counting individual photons rather than total flux. Results are given in units like photons/cm² or photons·cm⁻²·sr⁻¹ instead of W/cm².
These instruments are limited to low irradiance because the detector’s readout electronics have a time resolution in the megahertz range, and the maximum irradiance is also constrained by the detector’s throughput and gain. For NIR, visible, and ultraviolet wavelengths, a photomultiplier provides sufficient sensitivity. In airborne or space-based remote sensing, photon counters are used at high energies (X-ray to far ultraviolet) due to the low radiant intensity of targets and the particle-like nature of light at those energies; radiometers are typically used from visible through radio frequencies.
Photography
Photometers determine correct exposure in photography. Modern cameras usually have built-in photometers. Because illumination varies across a scene, advanced photometers measure light intensity in different parts and use an algorithm to set the best exposure, adapting to the intended picture type (see Metering mode). Historically, separate exposure meters served this purpose; advanced models could measure light from the whole scene, from specific elements to optimize exposure for key parts, or measure incident light with an integrating adapter.
Quick Facts
- Field
- Photometry
- Known for
- Measuring luminous flux
- illuminance
- or luminance
- Types
- Rumford's photometer
- Ritchie's photometer
- extinction-of-shadows photometer
- modern electronic photometers
Facts from the source article.
The Dawn of Measuring Light
Before the advent of electronic sensors, the task of quantifying light fell entirely to human perception. Practitioners would position a device between a test source and a reference source, adjusting until the eye perceived equal brightness on both sides. The inverse-square relationship between distance and illuminance then allowed the relative fluxes to be calculated. By 1861, three distinct mechanical approaches had become standard in laboratories.
Rumford's shadow photometer exploited the observation that a more intense light casts a deeper shadow; by matching shadow depths on a sheet of paper, the ratio of intensities could be read directly from the ratio of squared distances. Ritchie's design used a narrow box containing an angled wedge of white paper, with the observer peering through a small tube to judge when both illuminated faces appeared equally bright. The third method, based on the complete extinction of a shadow when a second source was introduced at the correct distance, was regarded as the most precise of the three.
From Photons to Currents: Modern Detection
Contemporary photometers have largely replaced the human eye with semiconductor and vacuum-tube detectors. The most common sensing elements are photoresistors, photodiodes, and photomultiplier tubes, each converting incident light into a measurable electric current. For spectral analysis, the device may route light through a filter or a monochromator before detection, enabling measurements at specific wavelengths or across a full distribution. A more specialized class of instrument performs photon counting, registering individual quanta of light rather than aggregate flux.
Because the readout electronics operate in the megahertz range and the detector's throughput and gain impose hard ceilings, these counters are restricted to low-irradiance environments. In the near-infrared, visible, and ultraviolet bands, a photomultiplier provides the sensitivity needed. In airborne and space-based platforms, photon counters are deployed at the extreme high-energy end of the spectrum, from X-rays to far ultraviolet, where the particle-like character of radiation makes discrete counting more practical than the wavelike techniques used by radiometers in the visible, infrared, and radio ranges.
Serving Science, Industry, and the Photographer
The photometer's reach extends far beyond the laboratory bench. In photography, it remains the tool that establishes correct exposure; modern cameras integrate the sensor directly, while earlier systems relied on a separate handheld exposure meter. Advanced models sample light from multiple zones of the frame and apply an algorithm tailored to the intended image type, ensuring that the most important elements are properly exposed. In analytical chemistry, absorption spectroscopy uses the instrument to calculate the concentration of dissolved substances, while infrared spectroscopy reveals molecular structure and atomic absorption spectroscopy quantifies metal content in solution.
In the paint and coatings industry, a reflectance photometer illuminates a surface with white light and measures the reflected spectrum through a monochromator, providing an objective, wavelength-by-wavelength characterization of color that replaces subjective visual assessment. Across all these fields, the underlying task is the same: translating the physical quantity of light into a number that can drive a decision.
The Eye as the First Detector
Long before any electronic component could sense light, the human eye served as the sole photometric detector. The working principle was deceptively simple: place a test source and a standard source on opposite sides of a reference surface, then adjust their distances until the observer judged the two illuminances to be equal. Because illuminance falls off with the square of distance, the ratio of the two distances immediately yielded the ratio of luminous fluxes.
A classic implementation used a sheet of paper bearing a small oil spot that made the paper slightly more transparent at that point. When the spot disappeared from view on both sides, the illuminances were matched. Ritchie's apparatus refined this idea with a purpose-built box: the observer looked through a narrow tube at an angled wedge of white paper, and the two lights were repositioned until both faces appeared equally bright. In every case, the instrument was essentially a mechanical aid that guided the eye to a perceptual null point, after which simple geometry completed the measurement.
Frequently Asked Questions
What exactly does a photometer measure?
It measures photometric quantities, most commonly luminous flux (total visible light emitted), illuminance (light arriving on a surface), or luminance (light emitted or reflected from a surface). These are distinct from raw radiometric energy and are weighted to match human visual sensitivity.
What historical types of photometers existed?
Nineteenth-century designs included Rumford's photometer, which judged brightness by comparing the depths of shadows cast by two sources; Ritchie's photometer, which relied on achieving equal illumination across a surface; and an extinction-of-shadows variant.
How does a modern photometer convert light into a reading?
A modern photometer uses a photoresistor (or similar light-sensitive element) to transform incoming light into an electric current. That current is then processed and displayed as a numerical photometric value.
More in Science & Measurement
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Photometer (CC BY-SA 4.0).
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