NPL's Optical Technologies group has established a world class reputation for surveying commercial and industrial lighting installations and for designing and making high accuracy light measurement instruments.

Throwing Light on the Use and Abuse of Luxmeters

Luxmeters are a common sight in industry and science. Lighting engineers use them not only to check illuminance levels in offices and factories but also to make performance checks on the lighting used in transport, photographic and film studios, hospital operating theatres and so on. There are even applications in the aerospace industry, for materials testing and the design of cockpit displays in aircraft. However, it is seldom appreciated that while the digital displays on these instruments often indicate fractions of a per cent, the accuracy of meters used in most practical applications is seldom better than 10%.

The principal reason for this is the difficulty in matching the spectral response curve of the detector to the eye's response function, V(λ). This means that even if the meter has been accurately and traceably calibrated using a tungsten lamp - as recommended in most specifications - errors will occur when it is used to measure other types of source with a different spectral distribution, such as fluorescent lamps.

The Optical Technologies group has extensive expertise and specialist facilities available to address this problem. These range from advice and training in the optimum use of luxmeters, through accurate calibration of customers' meters to allow fully for the effects of the spectral mismatch, to the supply of special individually- designed photometers for applications where the very highest accuracies are required.

Our scientists have made the accuracy of your luxmeter readings their priority. Call them in before you expose your project to luxmeter reading errors.

Our Photometers are based on the Primary Standard

The Optical Technologies group is able to supply special, individually designed photometers based on those developed for the realisation of the SI photometric base unit, the candela. They are constructed from a silicon photodiode and four element glass V(l)-correction filter, all contained within a temperature controlled housing capable of maintaining a temperature of 30 °C in ambient temperatures between 18 °C and 29 °C. Each photometer is supplied complete with its associated temperature control and photocurrent read-out unit.

The V(l)-correction filter for each photometer is individually designed, based on the measured silicon photodiode responsivity and the spectral transmittance of the glasses used. Generally, the filter is optimised for the photometric measurement of 'white light' sources (tungsten lamps, fluorescent lamps, high pressure sodium lamps etc) but optimisation for other spectral emission spectra, for example red LEDs, is also possible on request.

NPL photometer response Vs wavelength 

For commonly encountered white light sources, the correction necessary to allow for spectral mismatch is usually of the order of 1% or less and can be neglected in most applications. Each unit is calibrated for spectral responsivity, allowing corrections for more highly coloured sources to be calculated, if necessary.

These photometers are intended for use on a photometric bench in a laboratory where measurement accuracy is the prime requirement, and are not suitable for field use. They are ideal for high accuracy measurements on quasi-point sources, being fitted with a well-defined limiting aperture from which the lamp-photometer separation can be accurately measured. The absolute illuminance calibration uncertainty (using a 2856 K tungsten reference) is: ±1% or better, with a typical change in response of < 1% per annum.

For further information, please contact: Martin Dury