Sunday, August 14, 2011

Telecentric Lenses with Collimated Light: Part III, Spot Size (I)

[Note to readers: Here’s the third of four posts by Guest Blogger Spencer Luster. Spencer runs a business that specializes in telecentric imaging, and if you don’t know what that is, I suggest you read his articles and visit his web site,]

Last time we looked into shift and angular alignment of collimated light with a telecentric lens. This time we'll consider the size of the image spot (I) of the source (S) as compared to the size of the telecentric entrance pupil (E). [Please see Part II for earlier details.]

If (I) is the same diameter as (E), then the lens and the collimator will be perfectly matched in terms of angular sensitivity. The problem is that there will be no tolerance for misalignment. Any deviation, even due to vibration, will mean that (I) shifts position and some light will not enter the lens. This causes an overall signal drop.

There are two solutions: 1) Make (I) smaller than (E), or 2) Make (I) larger than (E). For the former case, the small (I) is free to "wander" within the boundary of (E) with no change to the overall light level. For the latter case, the large (I) can likewise "wander" while (E) still collects the same amount of light.

Figure 4 shows (I) being small because (S) starts small.

We could make (I) larger by starting with a larger (S), but another way is to use a shorter focal length (F1) collimating lens (C). Figure 5 illustrates this.

The size of (I) is proportional to (F2)/(F1). Of course you'll almost never know the actual value of(F2), but you can control (F1) if you're making the collimator.

So, is it better to have (I) larger or smaller than (E)? Like nearly everything in the universe, it depends. Small (I) means greater sensitivity, but it also means less light into the lens. For critical gauging—where low-angle reflection is a serious issue or extreme sensitivity to defects is required—small (I) is probably preferred. For less critical applications, or if the system is light starved, the higher collection properties of large (I) may be the better choice.

Come back for part IV in which we introduce: Collimated Dark! Dun, Dun, DUNNN!

Spencer Luster

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