Surface Reflectance Meter
Measurement of grey-scale
reflectance of surfaces.
 
 
 RM103 Reflectance Meter & Sensor Head
Reflectance meter model RM103, with the Oblique reflectance sensor head.

The RM103 Reflectance Meter is designed to measure the grey-scale reflectance of surfaces. A typical system includes the RM103 meter/controller base (with power supply), and a hand-held, self-illuminating sensor head (with cable). The meter is typically used in conjunction with two laboratory or factory calibration reference standards, chosen by the user. (The meter stores the calibration settings when off.) Model RM103 was designed as an improved version of model RM102, based primarily on practical use of the earlier model.
 
 
 RM102 Reflectance Meter with Normal Head
Reflectance meter model RM102, with the Normal reflectance sensor head.
 Calibrating the Meter
Calibrating the meter with a reflectance standard.
 Measuring a Book Cover
Measuring the reflectance of a book cover.
 Typical Plot of Instrument Readings
This graph illustrates the typical range and linearity of the reflectance meter, measuring grey, diffuse, laboratory-grade reflectance standards. These instrument readings were taken with a set of eight such standards (shown below). In this case, two of the standards, near 0.1 and 1.0 reflectance (specified by the manufacturer in the 400-700nm range), were measured and set as the calibration points for the reflectance meter. The other six plotted points, between and below the calibration points, are those measured using the remaining standards. The meter interpolates (and extrapolates) using any two calibration points, and typical nonlinearity* is found to be about +/- 1% over the entire instrument range.

 Reflectance Standards

* Nonlinearity is taken to be the percentage ratio of the largest instrument measurement error to the range of the instrument. The instrument error for a specific measurement point is computed as the difference between the measured value and that predicted by a straight-line (least squares) fit through all available measurement points distributed across the instrument range. In this case 8 measurement points span an instrument range of about 1.0.
 
 
Oblique Reflectance Sensor Head
For measurement of diffuse reflectance of glossy surfaces.
 Oblique Sensor Head
The Oblique sensor head facilitates diffuse grey-scale reflectance measurement of surfaces that are particularly specular (glossy), such as those that are polished or covered with transparent coatings. The offset measurement angle (22.5º) of the sensor head reduces the troublesome effect of specular reflection interference, encountered typically with near-perpendicular reflection from shiny coatings on otherwise darker, diffuse surfaces. This sensor has a reflectance range similar to that of the Normal head noted above, and typical nonlinearity is found to be less than +/- 1.5% with diffuse reflectance standards. Under conditions where measurement is hampered by irregular glossy surface coverings, say wrinkled plastic wrap, nonlinearity has been found to be typically less than +/- 2.5%.
 
 
Additional Items
Introduction from the Manual
 
General Description and Operation
 
The RM103 Reflectance Meter is designed to measure the grey-scale reflectance of surfaces. A typical system includes the RM103 base unit (with power supply), and a hand-held, self-illuminating sensor head (with sensor cable). The meter is typically used in conjunction with two laboratory or factory calibration reference standards, chosen by the user. (The meter stores the calibration settings when off.)

The sensor head receives power from the base unit, illuminates the surface to which it is applied (via white LED’s), senses the level of light reflected, and sends this signal back to the base unit. The base unit computes the reflectance by linear interpolation, based on two calibration points acquired at any time previous (on the current session or at any time before), and outputs the result on a multi-line display on the front panel. The measurement operation is essentially continuous, and the computed reflectance is updated and displayed a few times each second.

Using the sensor head in a hand-held fashion, it may be applied to various surfaces, with the reflectance of each surface being displayed without further user action. One may also place various samples onto the open end of the (Normal) sensing tube, while the sensor head rests on its ‘back’. The front panel keys are required only to select display settings, or to accept user inputs during set-up or calibration.

System set-up requires the connection of the sensor head to the base unit, using the sensor cable, and attachment of the external power adapter (9 Vdc @ 500mA) to the base unit. It is recommended that for maximum accuracy, the system should be allowed to stabilize (warm-up) for at least 30 minutes after switching power on, and then calibration should be performed (which takes about 1 minute) immediately prior to the session of use. Experience has shown, however, that practical accuracy is typically obtained with any previously calibrated unit, as long as it is allowed to stabilize for about 5 minutes after power-up. The most reliable performance of the system is achieved when it is left continuously powered, and calibration is performed (or at least checked) whenever one begins a measurement session.