How to Focus Light to a Phototransistor
- 1). Read the data sheet for your phototransistor and find the size of the light-sensitive region.
- 2
Light detection is at the heart of communications, analysis, and security systems.Kim Steele/Photodisc/Getty Images
Determine the size of the input beam. Either measure the diameter of an existing beam or find the beam diameter from the system design. - 3). Determine the diameter of the light-collecting lens. If the system is very stable, the diameter of the lens does not need to be much larger than the input beam. If the beam will be moving, you need to understand the range of expected motion and make sure the lens is large enough to capture the beam, no matter where it is.
- 4). Select a focal length for the lens. You need to balance the physical space available -- against the optical quality you need -- against the expected change in beam angle. A longer focal length lens will be less susceptible to optical aberrations and will not require the distance between the lens and detector to be tightly controlled. On the other hand, a shorter focal length lens takes up less space and will stay aligned as the beam angle changes. A middle ground for the focal length would be a length between two and six times the diameter of the lens.
- 5). Place the phototransistor at the focal point of the lens. This is the point where the light is concentrated down to its smallest area. If the lens is not very good, the spot may not be very clean and symmetrical, but for many applications that won't matter. If you're working on a communication application, where you need a fast, clean detector response, you'll need to invest in a higher-quality lens.
