[] A measure of the visible light response of a fluorescent material to black light. It is equal to π times the luminance in candelas per square meter produced on the material, divided by the incident black light flux density in milliwatts per square meter. It may be measured in lumens per milliwatt.

Note: π was introduced to maintain the same numerical glow factor values when the system involving footlamberts was declared obsolete. π is omitted when luminance is in footlamberts and the area is in square feet.

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