Photometeric System Specification
Specifying a photometric system is best approached in three steps. First, evaluate the source to determine which measurement technique best applies. Then, select a detector and optical system (detector head) that suit the measurement. And finally, match the detector head to the particular electronics which provide the most effective user interface for the application.
Consider the Source
Common sense goes a long way in determining the right measurement for an application. After all, photometry is concerned with the relation of light to the human eye. So, the first question is: how will people be affected by the source to be measured?
For instance, measurements of ambient or environmental lighting are concerned with people‘s ability to read print or safely see objects in an area. It is not the power of a particular source that is of concern, but rather how well the source lights the area of interest. For this reason, lighting for the outdoors, offices, factories, and photography are measured in terms of illuminance.
However, if in the same room or space one wished to determine the brightness of walls, fabric, or painted surfaces, the measurement changes altogether. Because now the amount of reflected light received by the eye is of concern. Since all of these surfaces are diffuse and relatively uniform, a luminance measurement would best apply.
Electronic displays such as CRTs, avionics, and automotive panels are incident directly upon the eye too. But alpha-numeric characters and line detail are generally small. So the measurement system‘s field-of-view must be limited or focused in order to measure only the lighted portions of the display. This is, by definition, a luminance measurement. So display brightness is usually specified in footlamberts.
Lamps are used in so many applications that it is impossible to define just one way to measure them. As previously mentioned, lamps and lamp systems for area lighting (rooms, streets, stadiums) call for illuminance measurements. But in automotive exterior lighting, headlights are usually measured for illuminance, taillights for luminance. There are a number of miniature, lensed lamps on the market, and since their divergence is of concern, they would be measured for luminous intensity. Incandescent and fluorescent lamp manufacturers specify products in terms of luminous flux (or the radiometric equivalent, watts) since these will be placed in fixtures meant to diffuse and measure their total output.
Lasers and LEDs also require a careful approach. They are measured in radiometric terms for scientific applications. But when their potential damage to the eye is of concern, they would probably be measured for luminous flux. A lensed LED, however, is a divergent, though directional, source. Luminous intensity would best characterize it. But with surface or edge emitting LEDs, emission as a function of surface area is significant. This describes a luminous exitance measurement.
Luminous energy measurements apply to any periodic source. Pulsed LEDs, photographic flash units, strobe lights, arc lamp systems, and rotating or scanning lights are several examples of sources whose flux is time dependent.
For Additional Information on Photometry and Photometric Testing Visit our Light Measurements Guides and Tutorials Page
Selecting the Right Detector Head
The measurement type dictates your choice of detector head assemblies.
UDT Instruments offers a modular photometric sensor-head design approach. In all cases, a silicon photodetector, detector housing, and photometric filter assembly are provided. And for those luminous flux measurements where all incident light is collimated or focused onto the detector, this simple head will suffice.
However, if flux levels exceed 70 lumens per square centimeter, the detector may become saturated, and its output nonlinear. In such instances, attenuation is recommended. Neutral-density filters, apertures, or integrating spheres achieve the desired effect. The correct selection depends upon the amount of attenuation desired: it should be enough to avoid detector saturation, but not so much as to lose sensitivity and dynamic range.
The simple detector/filter arrangement is also effective for ambient measurements if all light is at normal incidence. But when off-axis light, such as from windows and peripheral sources, contributes to the total flux, a cosine diffuser is needed.
In addition to being widely applied by lamp manufacturers, integrating spheres are useful for measurements of small divergent sources like lensed LEDs or miniature lamps. These can be inserted right into the sphere‘s entrance port to ensure that all light is collected.
Luminance measurements require a prescribed sensor-head field-of-view. The size of the source in the measurement-field plane, and the sensor-to-subject distance determine the angle. With large, but close fields, a simple baffle (steradian shade or aperture) will do. But small images, such as those on CRTs or avionics, call for a lens system, as do measurements at a distance. A variety of lens assemblies and optical accessories are available from UDT Instruments, to accommodate most any luminance measurement, whether microscopic or telescopic.
UDT Instruments offers a wide range of optical accessories for out-of-the-ordinary measurements. These include: fiber optic probes, for convenience in measuring sources hidden in hard-to-reach places; LED measurement systems specific to either segmented or discrete LEDs; low-profile sensors for slipping into tight spaces, such as in photolithography exposure systems; and a variety of sensor heads customized for CRT luminance measurements.
Choosing Electronics Matched to the Application
The light sensor in each UDT Instruments photometric head is a silicon photodiode. Though sensor size may vary, the output will in all cases be a low amplitude current signal. This signal will be converted into a voltage by a transimpedance amplifier circuit, and then used according to the requirements of the particular application.
Your choice of electronics depends upon the answers to a few basic questions:
- Is field portability needed?
- Will the instrument be interfaced with a computer?
- Is a visual display desired, or will an analog output suffice?
- Will more than one measurement be conducted concurrently?
UDT Instruments offers photometer controllers and electronic amplifiers that satisfy any combination of answers to these questions. The instruments range from simple analog amplifiers and hand held photometers, to multichannel computer-controllable laboratory instruments. Versions are available which suit most any budget.