I need a light source and a detector to measure distances ranging from 0 to 3 yards with a resolution of < 1 inch.
There are many ways to measure distance optically - the ideal method will depend on your use case. A short overview:
(1) Time of flight, aka ranging: a measurement is made to estimate the time it takes for emitted light to travel to a target and back. This is typically done with pulsed lasers as an emitter and requires electronics with a bandwidth of several hundred MHz, up to about 1 GHz. The detector should be a photodiode with a low junction capacitance (in your use case, probably as low as 5-10pF), or would need to be an avalanche photodiode. Typically in these cases external optics are required, since photodiodes with low capacitance tend to have very small photoactive area (sub 1mm^2). The effective resolution will depend on the bandwidth of the photodiode+analog front end.
(2) Interferometers: in a Michelson interferometer, a beam of coherent light is split into two separate beams. One beam travels a fixed distance and back to its source, determined by the placement of a mirror in its path. The other beam reflects off of an object and returns, its phase modified as a function of the distance of the object. The distance of the object can be calculated from the interference pattern that occurs from the addition of the two out of phase beams of light when they recombine. This may be impractical for measuring distances of 3 yards, as the optics would need to take up a lot of space, but is good to be aware of. Accuracy will be very high for this method.
(3) Measurement of light intensity: in some cases, if the reflectivity of the object you are measuring is known or is a constant, distance can be approximated by measuring the intensity of light reflected off of the object. This can be done with a photodiode and a high-power LED, such as the OSRAM SFH4259. The upside to this is simplicity: analog front ends such as the ADPD105, common PIN photodiodes and infrared LEDs already exist and are simple to put together. The downside is that distance here is not directly measured but is only an estimate. Light intensity (power/area) emitted from an ideal point source falls off proportional to 1/(r^2), where r is the distance from the source. If it is scattered off of an object, again modeled as another point source, it falls off again proportional to 1/(r^2) from the object back to the detector. The total light intensity returned to the detector as a function of distance is then 1/(r^4), which is a very nonlinear function. It will be very sensitive to changes in distance when distance is close to zero, and become progressively less and less sensitive as distance increases (real-life is slightly better than this, as most emitters/scatterers are not really point sources, but this should give some idea of this method's complications).
(4) Triangulation: distance can be measured by looking at the angle of light reflected off of an object, incident on two locations some distance apart. This may be done with two ADUX1020s - a sensor that detects the angle of light incident on its photodetector. The source in this case could be a high power LED such as the OSRAM SFH4259. This method has the advantage of giving a linear and high accuracy measurement of distance if done right, and it is relatively simple. The largest challenge here will be getting enough light to the object.
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