Light meters translate light into exposure settings or illuminance readings. Older meters used selenium or CdS cells; modern handheld units mostly use silicon photodiodes or similar semiconductor detectors with amplification and processing. Meters measure either reflected light (what the camera sees) or incident light (what falls on the subject). Flash meters, color meters, densitometers, and spectroradiometers address specific studio and color-critical needs. Calibration and task-driven choice matter: incident meters for reliable subject exposure, spot meters for precision, and spectroradiometers for color accuracy.

What a light meter does

Light meters measure the intensity of light so you can control exposure or verify illumination levels. Photographers use them to choose shutter speed and aperture (or confirm a camera's settings). Lighting designers and facility managers use meters to check that luminance or illuminance meets safety or design criteria.

How meters measure light

Sensors fall into a few families. Older meters used selenium cells (photovoltaic) or CdS (cadmium sulfide) photoresistors. Selenium cells can generate a small voltage directly; CdS cells change resistance and typically require a power source and amplifier. Modern handheld meters more often use silicon photodiodes or similar semiconductor detectors with electronic amplification and microprocessors to compute exposure or lux values.

Meters report either luminance/illuminance (lux or foot-candles) or camera exposure settings (aperture, shutter, ISO). Some display a needle or an LCD readout; many now add memory, averaging functions, Bluetooth, or USB connectivity.

Reflected vs. incident meters

Reflected-light meters read light bouncing off the scene. They are what most built-in camera meters use and give a reading based on the subject's reflectance. That makes them fast but can be fooled by very bright or very dark subjects.

Incident-light meters measure the light falling onto the subject. You take an incident reading by placing the meter at the subject's position and pointing the meter's diffuser toward the camera. Incident readings are independent of subject reflectance and often more reliable for controlled lighting.

In landscapes or distant scenes where you can't place a meter at the subject, photographers use spot or reflected metering and compensate for known conditions.

Flash, color, and spectral measurement

Flash meters measure the brief burst of light from strobes and flashes; pro units can measure peak output, sync timing, and calculate guide numbers. Color meters and densitometers (and higher-end spectroradiometers or spectrometers) evaluate color balance, density, and spectral power distribution for accurate color reproduction in photography, printing, and display calibration.

Practical choices and modern options

Many cameras offer accurate built-in metering (matrix/evaluative, center-weighted, spot). Professional still and motion-picture photographers still use handheld meters for scene-by-scene control, especially when lighting is complex.

Smartphone light-meter apps provide convenient approximations but rely on the phone's camera and internal processing; they are useful for quick checks but usually lack the calibration and spectral response of dedicated meters.

Calibration and standards

Manufacturers recommend periodic calibration or verification against a certified reference. An older standard often cited for photographic exposure meters is ISO 2720:1974; check current standards and calibration services for up-to-date guidance. 1

Buying guidance

Choose a meter by task: incident and flash meters for studio work, spot meters for precision scene metering, spectroradiometers for color-critical work, and simple lux meters for facility lighting checks. For occasional or casual use, smartphone apps can suffice, but professionals generally prefer a calibrated handheld meter.

  1. Verify current common sensor types used in new handheld light meters (prevalence of silicon photodiodes vs CdS/selenium).
  2. Confirm the current status and any revisions of ISO 2720 (Photography - Electrometric exposure meters) or relevant modern standards for photographic light meter calibration.
  3. Optionally, verify claims about smartphone app accuracy compared with calibrated meters for specific phone models if required.

FAQs about Light Meters

What's the difference between reflected and incident metering?
Reflected meters read light bouncing off the scene and depend on subject reflectance. Incident meters measure the light falling on the subject and give readings independent of the subject's brightness.
Are smartphone light-meter apps accurate enough?
Apps can give useful approximations for quick checks, but they rely on the phone camera and processing and usually lack the calibration and spectral response of dedicated meters. For critical work use a calibrated handheld meter.
Do modern light meters still use CdS or selenium sensors?
Selenium and CdS sensors were common in older meters. Today, most new handheld meters use silicon photodiodes or similar semiconductor detectors with electronic amplification and processing.
When should I use a flash meter?
Use a flash meter to measure strobe output, confirm exposure for a given aperture/ISO, determine guide numbers, or check flash consistency and timing in studio photography.
How often should I calibrate a light meter?
Calibration intervals depend on use and manufacturer guidance. Professionals typically verify calibration annually or after heavy use, or when precise color or exposure control is required. Check current calibration standards and services.