Mechanical Center vs Optical Center
When LEDs are placed into a typical test fixture, it is usually placed in such a way as to assume the light is emanating from the device's mechanical center. This is frequently not the case. The optical center may deviate 5° or greater from the LED mechanical center due to die placement, packaging tolerances, etc. This may not present much of a problem when measuring devices with a wide viewing angle (>40°), however for narrow angle devices the difference in readings may be very noticeable. The CIE recommendation is to use the mechanical axis of the LED rather than the optical axis as the measurement reference.
