3.2 Back Focal Length

The holder should have clearance for the lens Back Focal Length (BFL). Back focal length of the lens is the physical distance between the bottom of the lens barrel to the top of the sensor in air. The concern is that the holder might not have enough clearance to allow the lens to screw in all the way to come in perfect focus. It is also possible in some rare cases that the holder does not have enough thread length to allow the BFL of the lens. Sensor cover glass will have a refractive index greater than 1. So the equivalent path length in air has to be considered as BFL for the actual physical distance between the lens and the sensor package glass top. Example calculations follow:

Sensor package glass thickness, d: 0.5 mm

Sensor package glass refractive index, µ: 1.5

Lens BFL (specified in data sheet): 3.03 mm

Path length required = 3.03 mm

Path length inside the cover glass = µ × d = 0.75 mm

Net physical distance from sensor = BFL – (µ – 1) × d

Physical distance from cover glass = BFL – µ × d