DIY optics notes

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That is, the kind of optics that are still simple enough to let you use them in DIY projects.

Some theory

Geometric optics

Terms like 'geometric optics' and 'ray optics' is usually when analysing the propagation of individual rays, and how to describe what a collection of such rays does, e.g. describing the workings of one or a few lenses.

See ray tracing diagrams.

Much of that involves images.

The contrast is mainly with nonimaging optics, which concerns itself more about getting most of the light somewhere else (sometimes involves specifically not showing an image of the light soure)

Light sources types


Some lenses





Fuzzy shadow projections

Nonimaging optics Non-imaging optics

Some examples

Overhead projector

Stage lighting

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Stage lighting can useful to study for some basic cases of (mostly non-imaging) optics - and sometimes to buy because it does what you want.

PAR spot

"Parabolic Aluminized Reflector"

(sometimes "Parabolic Anodized Reflector" - close enough, the aluminium is typically anodized)

Frequntly called PARcans, pointing out that the fixture itself is a can that does little more than house the light (and sometimes transformer) - the light itself has the reflector and lens.

which makes the cans rather cheap and bulb replacement pricy relative to the can
  • fairly directional beam, throwing a pool of light with unfocused edges
angle is a property of the bulb, so changing the angle is done by changing the bulb
a can may only be useful for a range of beam angles (verify)
  • optional coloring with gels mounted on the front
LED variants can mix colors (but rarely produce enough light for larger stages)

The number after PAR (e.g. PAR16, PAR20, PAR30, PAR36, PAR38, PAR46, PAR56, PAR64) is the diameter, in units of eights of an inch

so e.g.
PAR16 is ~5.1cm
PAR36 is ~11.4cm
PAR56 is ~17.8cm
PAR64 is ~20cm
16 and 20 are lowish-light, and often use no lens because they are used for nearby things (e.g. lighting art). They're known as birdies because they are sub-par. Har har.
also correlates to bulb sockets
PAR16 may well be GU10
PAR20 may well be E27 or GU10
PAR30 may well be E27
PAR36 often seems G53 or screw terminals
PAR38 can be E27
...but always check, because there's usually more than one type a sizes
PAR64, being common in stage and studio lighting, is sort of its own thing, but may well be GX16d (a.k.a. Mogul end prong(verify))


Usually means "something with a relatively narrow throw that lights the discoball from not too far away"

ERS spot

ERS spot (Ellipsoidal Reflector Spotlight), profile spot (UK), Leko (US; Leko was once a producer of these things)

  • There are varied designs, with similar purposes
  • can be focused for a hard or sharp edge
Cannot significantly change the beam size, though.
  • optics more complex than PAR, and more like a projector
can project a sharpish outline image via a gobo
and have internal shutters that essentially trade light yield for sharpness

Optics are basically:

  • an elliptic reflector,
  • shutters (instead of barndoors, because they are better at this job)
  • a gobo slot (usually)
  • two (plano-)convex lenses, the first often soon after the ellipsoid's second focal point to tame to beams to nearly parallel, the second for ?, and in a movable barrel to control size and softness(verify)
The further the lenses (= the longer the barrel), the narrower the projected beam Template:Comment(also depends on the reflector).
  • a color filter holder at the end isn't unusual either.

Usually very simple lenses, meaning you'll get some color fringing.

A smaller effective aperture reduces this, hence the shutters.

Followspot / spotlight

A spot meant to be manned by a person to follow an actor on stage.

Can give a sharp edge, though frequently softened somewhat.

Is like an ERS, but

  • easier to point (typically longer, on a pivot, and with long handles on the side)
  • easier to narrow with an iris
  • if coloring/gobo is present, these are easier and control/change

See also:

Scoop / ERF / Floodlights

Scoop / ERF (Ellipsoidal Reflector Floodlight)

  • wide angle, meant for general lighting in a large-ish area
  • lamp in (shallow) reflective dome

Other floodlights are often similar.

Fresnel lantern

A small light source, a reflector, and a nearby fresnel lens, manually focusable between flood and (some amount of) spot.

Small design due to one lens doing all the work.

Typically soft light - in part because fresnel lenses are often stippled on their flat side to avoid seeing the physical pattern projected.

No gobo

Plano-convex (PC) spot

Also known as Prism-Convex, and somewhat more specifically, Pebble-Convex.

Have been uncommon in the US and in general for a while now.

Comparable to a fresnel lantern (also in that there is rough focus-style control of the angle). differences:

  • didn't apply the glass-saving design trick the fresnel lantern is named for.
  • Basic PC lenses give sharp shadows (sharper than fresnel)
...yet often enough this is softened by using a pebbled lense
  • The thick lenses seemed to take more light away than fresnel does




See also



Total internal reflection

Frustrated Internal Reflection

A setup where total internal reflection happens in the usual case, but not if something opaque touches the surface.

You've probably seen this happen while drinking glass of water. Also used in fingerprint scanning, and a few multitouch screen designs.