- Measuring range
- 0 to 200,000 lx on the four AS instruments; 1 to 100,000 lx on the ST instrument
- Resolution
- 1 lx and 0.1 degrees Celsius on the AS instruments; 0.1 lx on the ST instrument
- Accuracy
- plus or minus 4 percent of reading plus 10 digits at or below 10,000 lx and plus or minus 5 percent plus 10 digits above it; plus or minus 3 percent plus 5 lx on the ST instrument
- Sensor
- Silicon photovoltaic cell with a colour-correction filter
- Sensor head
- Rotates 180 degrees to the right; the AS823 uses a separate probe on a lead of up to 2 m
- Units
- Lux and foot-candle, switched from the keypad at 1 fc = 10.76 lx
- Sampling rate
- 2 readings per second on the AS instruments; about 0.5 s on the ST instrument
- Display functions
- Data hold, maximum and minimum capture, overload indication, backlight and automatic power-off
- Supply
- 3 x 1.5 V AAA on the AS803F, AS803 and ST8813; a 9 V battery on the AS813 and AS823
- Body
- 163 x 49 x 25 mm and 85 g on the AS803F and AS803; 224 x 66 x 35 mm and 169 g on the AS813; 160 x 66 x 35 mm and 142 g on the AS823
An illuminance figure is agreed between two parties as a number on a page and then has to be produced on a site, and the gap between those two things is where most disputes about lighting live. A lux meter is the instrument that closes it, by putting a corrected sensor in the plane of interest and reporting what the eye would receive there.
The correction is what makes it more than a light-sensitive cell. A raw detector responds to radiant power, and the eye does not.
Product overview
The range covers five instruments across two designs. Four of them share a range to 200,000 lx, a 1 lx resolution, a head that rotates through 180 degrees and the same set of display functions, and differ in supply, body size, working environment and whether the sensor sits on the body or on a lead. The fifth is a different instrument with a ten-times finer resolution and a lower top of range.
The first thing to establish is therefore not which model is best but which family the duty belongs to. A check of desk, classroom or workplace lighting sits in a band where 1 lx is enough and 200,000 lx is generous. A check of a night-time or low-level condition sits at the other end, where 1 lx is too coarse to be a measurement at all.
How the sensor measures light
A silicon cell produces a current in proportion to the light falling on it. On its own that response is broader than the eye response, so it is filtered to approximate the photopic curve, and the resulting current is converted into a lux reading. The filter is the reason the instrument can be quoted against a lighting specification and a bare detector cannot.
Two geometrical effects decide whether the number means anything. The first is direction: the reading is the illuminance in the plane of the cell, so the cell has to sit in the plane of interest. The second is oblique light, which reaches the cell over a smaller effective area than it does a flat surface unless the assembly is corrected for it. The material does not state whether this correction is applied, which is one of the figures an acceptance test would normally require.
How to select the right light meter
Start with the band of illuminance to be measured, because it decides the family rather than the model. If the work includes low-level conditions, the finer resolution of the fifth instrument is what makes the difference between a reading and a rounding, and its lower top of range is not a limitation for that duty.
Then establish the environment. The entry model is specified to 0 to 40 degrees Celsius and the others to minus 10 to 50, which matters for an unheated site, a cold store or an outdoor check. After that the choice is mechanical: whether the sensor has to reach into a fixture or above a ceiling, which the separate probe settles, and whether a temperature reading in the same instrument saves a second visit.
Measurement limitations and selection considerations
The instrument reads the illuminance of a plane, not the luminance of a surface and not the output of a fitting. Those are three different quantities, and a figure quoted for one cannot be checked with an instrument that measures another. Where a specification is written in terms of a fitting, the measurement is usually of the task surface beneath it, and the plane and the distance are part of the procedure rather than of the instrument.
The scaled display is the other routine source of error. Above the digit count the meter multiplies the value and marks the multiplier, and a reading copied without it is out by a factor of ten or a hundred. Because the correction of the sensor is undefined in the material and the calibration arrangement is not stated, the instrument is offered as a field instrument for verifying and comparing conditions rather than as a reference against which another instrument is certified, until those two points are confirmed.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
