What a light meter is
A light meter, or lux meter, measures illuminance: the amount of visible light arriving at a surface, expressed in lux or in foot-candles. It is the instrument behind a lighting survey, a workplace check and a visit to a room that is too dim or too bright, and it reports a figure where the eye cannot, which is the point, because the eye adapts and the requirement does not.
The number is defined on a surface rather than in a direction, so how the sensor is held is part of the measurement. A sensor facing the ceiling in a room lit from the side reads something quite different from the same sensor held towards the task, and a reading taken at the wrong plane is not a fault of the instrument but of the survey. Most instruments settle this with a diffuser over the detector and with a sensor that can be angled or detached.
The type divides on how far the sensor has to travel and how much light has to be covered. An instrument with a fixed head is convenient and cannot reach into the space above a machine; one with a lead and a separate probe can be placed where the work happens and read from where the operator stands. At the same time the range has to stretch from the few lux of an emergency-lit corridor to the tens of thousands under daylight, and that span, rather than the resolution, is what separates the models.
How a light meter measures
The sensing element is a photodiode, normally silicon, whose output current is proportional to the light falling on it. On its own the element responds to a wider band than the eye does, including the near infrared, so a filter is placed over it to bring the overall response close to the curve that defines the lux. How close that match is decides whether a warm white source and a cool white source at the same true illuminance read as the same figure.
The measured current spans several decades between a dim corridor and direct daylight, so the instrument switches ranges or applies a scaling factor. On many models the display shows the significant figures only, with the multiplier indicated separately: a reading of 1455 beside a multiplication marker of one hundred means 145500 lux, and reading the digits alone is a large and common error. The multiplier is part of the measurement rather than an annotation.
The rest of the specification follows the same logic. Response time decides whether a reading settles before the surveyor moves on; the response as the sensor tilts away from the source falls off in a defined way, which is correct, because a surface really does receive less light at an angle; and a temperature channel, where present, is a separate sensor sharing the housing rather than a correction applied to the light reading.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Sensor mounting | A sensor fixed in the body, or a sensor on a lead that can be placed separately | Whether the measurement can be taken where the work is rather than where the surveyor can stand. For anything beyond a desk survey this is the most consequential choice on the list. |
| Range and scaling | From a few lux up to tens or hundreds of thousands, with automatic or manual range changing and a displayed multiplier | Whether one instrument covers an emergency-lit stair and daylight through a window. The multiplier has to be read with the digits, not instead of them. |
| Spectral response | An unfiltered element, or one filtered towards the response of the eye | Whether different light sources at the same true illuminance read as the same figure. It matters most with white LEDs, which concentrate much of their output in a narrow blue band. |
| Sensor geometry | A diffuser that corrects for the angle of incidence, and whether the head rotates or detaches | Whether an angled reading is corrected or simply lost. A head that cannot be turned forces the surveyor to estimate the plane of measurement. |
| Unit selection | Lux, foot-candles, or both on one instrument | Whether the reading can be compared directly with the requirement being applied. One is a metric unit and one is not, and the conversion is not a rounding. |
| Additional channel | Light only, or light and ambient temperature | Whether a second instrument has to be carried for the same visit. The temperature sensor shares the body and measures the air, not the surface being lit. |
| Power and autonomy | Primary cells or a rechargeable pack, with or without automatic switch-off | Whether a survey that runs over a day finishes on one set of cells, and whether the instrument protects itself from being left on. |
The sensor mounting is decided before the range, because a fixed-head instrument cannot be rescued by an impressive span if the head will not go where the reading is needed.
How to choose a light meter
| Decision | What settles it | What to ask |
|---|
| The requirement being applied | The illuminance the workplace or the task requires | The required figure, and the margin the installation is meant to hold above it, decides the range and the accuracy the survey needs. |
| Where the sensor has to go | Whether the measurement plane can be reached by hand | Above a machine, inside a booth or in a ceiling grid, a separate sensor on a lead is the difference between a measurement and an estimate. |
| The range to be covered | The brightest and dimmest conditions in the same survey | One instrument is convenient and two are sometimes necessary; the span decides whether a multiplier has to be read carefully on every reading. |
| The light sources present | Incandescent, fluorescent, LED or daylight | A white LED source is where a poorly matched filter shows itself and where two instruments begin to disagree. |
| Whether the reading has to be recorded | A survey sheet, or a report generated later | A hold function freezes a reading at the point of measurement; a logging function removes the transcription step and its errors. |
| Whether temperature is part of the visit | The comfort assessment that accompanies the lighting check | A combined instrument is convenient, but its temperature channel measures the air around the body and not the surface being lit. |
| How the instrument will be carried | The number of readings and the duration of the visit | A body that survives being set down repeatedly is a practical requirement and does not appear in a data sheet. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a light meter usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a light meter |
| Noise meters | Measure sound pressure where a light meter measures illumination. Both are used in the same workplace survey and against the same family of requirements, and neither can be substituted for the other. |
| Air quality monitors | Report what is suspended in the air rather than how the space is lit. A room can meet its illuminance target and still be unsuitable to work in. |
| Dust monitors | Measure particulate concentration in a process or an atmosphere rather than light. The two are occasionally confused because a dusty atmosphere also scatters light, but the quantities are unrelated. |
| Temperature and humidity data loggers | Record the thermal conditions of a space over time. They are often carried on the same visit as a lighting survey and answer a different question about the same room. |
| Weather stations | Measure ambient conditions at a fixed site over time, and their solar channel reports irradiance for an energy calculation rather than the illuminance a person works under. |
The boundary worth settling first is what is being measured: light arriving at a surface, or light leaving a source. A light meter answers the first, and a spectral or colour instrument is needed for the second.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The application and the requirement the reading will be compared against
- The illuminance range the survey has to cover, at both ends
- Whether the sensor has to be placed away from the operator, and how far
- The light sources present, and whether white LEDs are involved
- Whether the readings are transcribed by hand or logged
- Whether an ambient temperature channel is wanted on the same instrument
- The units the report is written in, lux or foot-candles
- Whether a calibration certificate is required with the instrument
