Human vision relies on light. After reflecting off surfaces, light enters into the eyes and travels through the cornea and pupil to produce an image on the retina. The eye is sensitive to a broad range of light intensity but at low light levels loses its ability to distinguish detail. That is why precision work such as measurement, surgery or assembly is best done under bright light.
Fatigue and mistakes can occur when people work under poor lighting, and industrial accidents are more common when light intensity is low. Moreover, good lighting determines how well people can view a show and take quality photographs.
Light can be defined as a form of electromagnetic energy that travels through space as a wave. Similar to X-rays and microwaves, those waves have a frequency and a wavelength. The difference is that humans have receptors that are capable of sensing energy with wavelengths ranging between 400 and 700 nm and convert it into images.
Separate wavelengths correspond to distinctive colors. Light with a wavelength around 635 nm is perceived as red, 525 nm is green and 420 nm is blue. Shorter waves are termed as ultraviolet and then X-rays and longer wavelengths are known as infrared (which is sensed as heat).
Heat-based light sources (“incandescent” sources) radiate electromagnetic energy across all wavelengths and that is why they are perceived as white. The temperature of the source decides the actual distribution of wavelengths within that light. Fluorescent lights will always appear white because of fluorescence from a coating on the tube or glass, and LEDs will produce light only at one specific wavelength.
How Light is Measured?
Similar to the filament of an incandescent bulb, a light source emits light in all directions. It effectively sits at the middle of a sphere of radiated light (which is why light units reference the steradian). “Luminous flux” refers to the total energy of all the light given off.
Candela is the fundamental unit of light and is nominally the light that is emitted by a single candle, or more precisely, “a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.”
One candela per steradian is termed a lumen, which is the measure of light intensity people are most familiar with. However, the most important aspect with regards to measuring light intensity is the number of lumens that fall on a surface, which is expressed as lux. Therefore, one lux is equivalent to one lumen per square meter, which relates to brightness to distance from the source. (In the US, light intensity is often expressed in the unit of foot-candles. One foot-candle is equivalent to one lumen per square foot).
To sum up, light output is expressed in lumens whereas light intensity is measured in terms of lumens per square meter or lux.
Situations Needing Light Measurement
The key reasons for measuring light intensity are to make sure that minimum standards of illumination are being met and also to establish appropriate exposure times in cinematography and photography. Set out below are four commonly encountered situations:
1. Ergonomics and Safety
Minimum illumination levels are advised for many environments.
In certain organizations, light intensity is only determined reactively, usually after a fall or other accident. Performing a lighting survey and thus documenting light levels across the workplace is a more prudent method. An improvement plan can be implemented if areas are found below the minimum acceptable levels.
2. Photography and Cinematography
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Photography studio
Undoubtedly, light intensity is at the core of photography. Low light will force a photographer to open up the lens aperture or increase exposure time, and sometimes both. Whilst many contemporary cameras are integrated with light metering, it is still useful to know light levels around the subject, particularly for portrait or studio photography. In addition, knowing light levels helps ensure the reproducibility of a shot which is kind of a major concern in cinematography. A cameraman can measure light levels to produce reliable results and thus maintain continuity.
3. Weather Monitoring
Although many light meters are configured for incandescent light, they are still handy for providing comparisons outdoors. For instance, a meter could create records demonstrating the difference in intensity between the winter and summer solstice. The optimal location for each panel can possibly be determined by mapping the light intensities in an area designated for solar arrays. Identifying areas of lower light intensity within a greenhouse can benefit people involved in agriculture.
4. Theater Set and Interior Design
An effective way to direct an audience’s attention lies in the differences in light intensities. A set designer may want a specific actor or prop cast in shadow for a particular scene and highlighted for the next. In a similar way, an interior designer will use differences in light intensities to create a specific look and feel. In addition, establishing light levels not only helps ensure the reproducibility of a particular look and feel but also confirms that there is adequate light for the audience to visualize the actors’ features.
Light Measurement Technology
Light falls on a sensor where the photons’ energy is changed into electrical charge. More charge will build up if more light strikes the surface. The two are correlated in general terms. A calibration in the measurement electronics changes either voltage or current to a lux value.
The human eye complicates this further. It has greater sensitivity to green although not equally sensitive to all wavelengths of light. Therefore, if equal intensities of green light and blue light fall on a meter, whilst the raw lux value might be the same the human observer will be able to perceive more of green light. In order to deal with this, lux meters are configured to anticipate light with the spectral distribution of domestic, tungsten-filament lighting.
This is referred to as CIE standard illuminant A and modifies the raw intensity measurement to better correlate with the human perception of brightness. It is recommended to use CIE standard illuminant A in all applications where incandescent lighting is used.
Light Measurement Meters
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Indoor work environment
Rugged Handheld Environmental Meters for Light Measurement and RPM are easy-to-use, handheld instruments designed for measuring light intensity. These instruments are based on the CIE Standard Illuminant A and are suitable for use in incandescent lighting areas. They will provide a reading under fluorescent lighting with a small error with a measuring range between 1 and 200,000 lux (0 to 18,580 foot candles).
These units are ideal for anyone who needs to validate light levels in indoor work environments, for interior design, theater set design, cinematography and photography. The instruments can be used outdoors where comparative ratios or values are adequate but should not be depended upon for accurate intensity values because of its CIE calibration.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.