Light meters are not only for photographers and movie makers. Today they are an important part of the arsenal of lighting engineers, building services professionals and facilities managers who want to be mindful of regulations. PASS managing director Barry Atkins looks at various types of light level meters from basic to advanced.
It is important to measure light and lux levels in the workplace because low light levels may cause fatigue, muscle strain, and impaired work productivity, while excessive light levels like glare and reflected light can affect employees’ vision.
The Health and Safety Executive (HSE) specifies certain types of illuminance and lighting equipment for different work situations. The Health & Safety at Work Act of 1974 includes a duty to provide lighting to ensure that work can be done safely, and that the health and eyesight of employees are not put at risk.
Regulation 8 of the Workplace Regulations Act 1992 covers lighting, including emergency lighting, and states that employers must ensure that the workplace has ample suitable lighting, with a preference for daylight where practicable.
Light meters will typically provide a luminance reading of either lux or foot-candles. Lux, the European standard of light measurement, is a unit of illumination of one lumen on a square metre which is one metre away from a uniform light source. Similarly the US foot-candle measurement standard is a unit of illumination on a square foot one foot away from the light source and corresponds to 10.76 lux.
Recommended light levels start from 20 lux for public areas, up to 200,000 lux for prolonged and exacting visual tasks of small pieces at low contrast. Offices are somewhere in the middle at 250-500 lux, while supermarkets may need lighting levels to be nearer to 750 lux. A hospital operating theatre may necessitate up to 1,500 lux.
In food production, over or under exposure to light means plants may not grow properly. Light meters also measure light pollution, a problem that becomes increasingly prevalent with more reliance on artificial light.
Artificial light readings can be used to analyse light pollution levels near places like airports. In the right hands, they can also enhance working environments and save energy on unnecessary lighting.
Basic light meters
The TestSafe TSLM3 digital light meter is suitable for a variety of light sources including fluorescent, metal halide, high pressure sodium and incandescent. It can measure 100,000lux or 10,000 foot-candles with an accuracy of ±3%, has an adjustable backlight and uses a silicon photodiode sensor with filter to calculate light readings.
The Martindale LM92 Lux Meter is designed for simple single handed operation with a backlit LCD display, peak hold and data hold features. The function switch is set to the desired range of lux or foot-candle units. If the lux or foot-candle magnitude is not known, the switch is set to the highest range and reduced until a satisfactory reading is obtained.
The sensor head has a 1.5m cable to separate the operator from the measurement location to avoid casting a shadow on it. For best accuracy, the measurement is repeated several times to ensure that the light source has remained stable.
Martindale explains that in light measurement, the inverse-square law states that the illuminance at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between the source and the point.
The cosine law states that the illuminance on any surface varies as the cosine of the angle of incidence: the angle between the normal to the surface and the direction of the incident light. The CIE photopic curve is an international standard for the colour response of the average human eye and may be used to convert radiant energy into luminous (visible) energy.
Intermediate light meters
In order to provide good quality lighting, luminous intensity in the workplace, hospitals, offices or schools has to fulfil specific minimum guidelines. The Testo 540 pocket light meter has a measurement range to 100,000 lux.
The 540 has been designed specifically with a sensor that is adapted to human eyes’ spectral sensitivity, making it suitable for measuring light intensity and ensuring that light levels are safe. A button provides maximum and minimum values and the device will operate for up to 200h on two AAA batteries.
Data logging light meters continuously display a real-time measurement and download measurements to PCs or Laptops to provide essential traceability. The Extech HD400 heavy duty light meter has a built-in USB port for easy data transfer to computers with a large backlit LCD display that makes it easy to see results.
It also features a large measurement range across four ranges of 40, 400, 4,000 and 40,000 foot-candles or 400, 4,000, 40,000 and 400,000 lux with basic accuracy of ±5% and resolution of 0.01 foot-candles or 0.1 lux.
The HD400 uses a precision silicon photo diode and spectral response filter to maintain measurement accuracy. The sensor of the device is connected to the unit by a 1m cable to allow the user to easily position the sensor where it can get the best reading on its 40-segment bar graph display.
Protected by a shockproof sheath in a compact case, the Chauvin Arnoux Physics Line CA811 light meter measures lighting levels by silicon photodiode to 20,000 lux on a 3½-digit display of 2,000 counts at a resolution of 0.01 lux and accuracy of ±3%. Featuring CIE spectral correction and incidence correction, it has a remote-controlled probe for all types of measurements.
Advanced light meters
Moving to more advanced light meters, the Extech EA33 EasyView light meter with memory for storage of test data is a handheld, portable light meter that can be used to gain accurate measurements of light to 100,000 lux within ±3% on multiple selectable parameters, even outdoors.
It utilises luminous intensity (candela) calculations and features a special ripple function that is designed to exclude the effects of stray light from the primary light source measurement on the overall measurement.
It can store and recall up to 50 measurements and includes relative or real time clock stamp. Other features include time hold measurement, relative in absolute value or percent deviation, and comparator function with high-low alarms. When the meter is turned on, the elapsed timer starts and shows days/hours or minutes/seconds using a button to toggle the view.
In addition to the normal mode of operation where light level is measured, the EA33 features five special modes: stray light inhibit which cancels out stray light from the light under test, time hold where the meter measures while timer counts down, real time clock to set the day and time for the internal clock, integral illuminance luminous energy measurement based on the rate of flow of flux expressed in lumen-seconds, and comparator mode where the meter compares its measurements to one high and one low limit.
The EA33 can also measure any photometric quantity on a time dependent basis. For example, the illuminance could be integrated over time to yield lux hours or foot-candle hours. As the sensor area is small, EA33 average mode can be used to measure the illuminance of larger surface areas where up to 99 points can be averaged.
The Testo 545 lux meter provides multi-point or timed mean calculation, the results of which it stores in up to 99 file locations, while a logger function allows up to 3000 readings. It offers fast printed documentation on site.
For all the bells and whistles, the Hioki 3664 optical power meter measures the light spectrum from Blue-Ray and High Definition DVD to near-infrared rays.
Trumpeted as ‘an advanced optical power meter to meet next generation needs,’ it is tool for R&D, manufacturing and equipment maintenance that incorporate laser light sources, such as DVD recorders, CD drives, copiers, and laser printers.
Specifically designed for blue-violet optical lasers that have low dependency on the incidence angle and flat wave sensitivity characteristics over its 405 nm bandwidth, the 3664 can be used for inspecting next generation optical pickup drive devices.
Users choose from four different optical sensors according to wavelength and sensor structure to meet application requirements. With accuracy of ±5%, it is suitable for making and testing optical pickup devices in combination with one of the sensors.
With a wavelength setting resolution up to 1 nm, up to 10 wavelength memory presets can be configured, including defaults for each optical sensor already stored in memory. Users may add up to six more custom wavelengths and call them up simply by pressing the lambda (?) key.
Data is uploaded by USB interface with software to program a PC to download captured data, configure and control the device through the USB interface.
Measuring light levels to comply with regulations is not expensive, starting from under £40 for a basic light level meter to perform low volume occasional measurement. For larger offices, plants and facilities or for more sophisticated applications, ‘you pays your money and you takes your choice’ as the saying goes.
For further information please visit PASS Ltd’s specialist light meters web page.