Tactile Tile Light Reflection

Tactile tile light reflection value (LRV) is a relative term.  By that, I mean that the amount of light measured reflecting off any surface varies with the intensity of the light falling on the surface. Light reflecting off Armor-Tile, concrete or any other surface will be different at sunrise, at mid-morning, at high noon, etc.  What matters, is the amount of reflected light off one surface, compared to the amount of reflected light off the adjacent surface.

“Light Reflectance Value” (LRV) is also a relative term used to describe how well a surface reflects light, while contrast is calculated in percent between the foreground color and the background color.  The current recommendation for detectable warning surfaces is a contrast of at least 70%.

Contrast is determined by the formula:

Contrast = [(B1-B2)/B1] x 100
B1= light reflectance value (LRV) of the lighter area
B2= light reflectance value of the darker area.

As Examples:
Assume the reflected light reading off the Armor-Tile is a LRV of 4200 Lux and the reflected light reading off the surrounding substrate is a LRV of 2300 Lux. The delta of 1900 divided by the lighter reading of 4200 will equal 45% reflectivity difference.

Assume the reflected light reading off the Armor-Tile is a LRV of 900 Lux and the reflected light reading off the surrounding substrate is a LRV of 3400 Lux. The delta of 2500 divided by the lighter reading of 3400 will equal 74% reflectivity difference.

Tactile tile light reflection readings can be taken with any number of reflectance meters.
The Minolta’s Flash Meter V (http://www.hotbuyselectronics.com/minolta_flash_meter_v.htm)  is geared for photography, but has the advantage of Lux as well as Candle Power readings, memory functions, and a “difference” feature which yields the delta between a standard reading and multiple (and continuous) subsequent readings.  Additionally, it can be made to show the continuous difference in +/– tenths of an f-stop, which can be read directly as a percentage difference from the standard.  The Minolta Flash Meter V sells for about $380.

Spectra makes a meter which may be considered also.  (http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=245894&is=REG).  It sells for about $372.

Any meter which is used, must be designed to measure reflected light, not ambient light. 
Ambient light is light which is falling on a subject.  Reflected light from a subject is what is important to measure when calculating reflectivity differences and contrast.

If either the Minolta or the Spectra (or similar) meter shown above were to be used for reflectivity readings, the white spherical dome receptor cover would need to be replaced with one designed for reflectivity readings.  The flat disc below is proper for reflectivity.  The domed disc is for ambient readings.  Domes and discs usually only require ¼ turn to remove and replace.
                     
It is important, when taking the light readings, to not interfere with the light falling on the subject.

Since measurement off domed subjects (like truncated domes) may catch a direct reflection of the sun, attention should be paid to minimize such occurrences.  This is similar to trying to photograph a car, yet the camera’s metering system is completely fooled by the brilliant reflection of the sun off the windshield.

Multiple readings should be taken to help eliminate anomalies.

If clouds are passing between the sun and the subject, inconsistent readings will result.

If the part of the meter which contains the sensor can swivel, then the sensor can face the sidewalk, while the user can still read the measurement.  Although a good meter should hold the last reading taken on its display, some meters can take continuous readings.  This would allow the user to see whether the measurement is fairly constant as the meter is moved over the surface, or if it is fluctuating wildly.  A swiveling head, or a sensor which is remote from the meter body, is a huge advantage.  All meters pictured above have this feature. I trust this helps.

By Ron O’Neill
Engineered Plastics Inc.
www.armor-Tile.com

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