Optics notes

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' usually refers to 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, because this is the analysis you want and need when doing accurate image projection.

You can contrast this 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) - consider parabolic reflectors in the spots you buy, and a lot of stage lighting.

Purposes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

relay lens

• 1:1, to extend the optical tube, e.g. in periscopes, endoscopes, sometimes to invert an eyepiece
• http://en.wikipedia.org/wiki/Relay_lens

field lens

• intends to change the size of the image, or provide telecentricity
• positive-powered
• after objective, before image
• http://en.wikipedia.org/wiki/Field_lens

collimator lens

• for lenses (and reflectors) this often means "getting the rays parallel"

• e.g. putting a light source in the focal point point of a positive lens

• in different applications you can also think of long tubes, parabolic reflectors, and some lens constructions in nonimaging optics (when used for lighting, you only need to get this approximately right to throw a get a lot of light in a bundle a few meters to a few dozen meters away - and arguably)

• and collimation also refers to having all parts on their optical axis, both in the sense of calibration, and of considering observers

• https://en.wikipedia.org/wiki/Collimator

• https://en.wikipedia.org/wiki/Collimated_light

Microscopes, telescopes, projection

objective lens (lens, mirror, or combination)

• gathers light to focus into an image
• for example
  • in everyday cameras (big to small image) we want a large focal plane, and need some trickery to minimize various optical aberrations
  • projector lenses are basically the same as camera lenses, in reverse (small image to big image)
  • in a lens telescope, the large front lens is the objective
  • in a mirror telescope, the back mirror is the objective
  • in microscopes it will be the last, a short-focal-length lens

https://en.wikipedia.org/wiki/Objective_(optics)

condenser lens

• Concentrates illumination.

Part of a larger system

e.g. in a microscope, you typically have at least: illumination source, condenser[1], sample/image plane, objective lens

note that the condenser can be non-imaging.

for example, in overhead projectors there is typically a (fresnel) condensing lens right under the sheet, which only really needs to care about sending the light it gets into the projection lens. It's perfectly fine if it's somewhat soft and milky (in fact that will hide the fresnel ridges, as they are near the image plane)

(there's more details to these designs if you want illumination and good focus, and may be aspheric dealies somewhat specific to the lamp, layout, and with some fine tuning)

http://www.tlsbv.nl/pages/optics/lens/spherical.htm

Nonimaging optics

https://en.wikipedia.org/wiki/Nonimaging_optics Non-imaging optics

Some examples

Stage lighting

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

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 PAR cans, 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 sometimes and lens).

which makes the cans rather cheap and bulb replacement somewhat pricier 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)

Diameter

The number after PAR (e.g. PAR16, PAR20, PAR30, PAR36, PAR38, PAR46, PAR56, PAR64) is the diameter, multiples of eights of an inch (3.175mm)

so e.g.

PAR16 is ~5.1cm

PAR36 is ~11.4cm

PAR56 is ~17.8cm

PAR64 is ~20cm

typical power for PAR36 and smaller is often ~30W-100W halogen, PAR56 around 300-400W, PAR64 around 500-1000W. ...except of course for LED variants.

16 and 20 are lowish-light, and rarely with a lens because they are used for nearby things anyway (e.g. lighting art). They're known as birdies because they are sub-par. Puns.

some correlationwith to bulb sockets

PAR16 may well be GU10 MR16 bulbs (that you may also be using around the house)

PAR20 may well be E27 or GU10

PAR30 may well be E27

PAR36 often seems G53 or screw terminals

PAR38 can be E27

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))

...but always check, because for most sizes there's more than one.

Angle

You may get some indication of angle, with acryonyms that were extended and changed and vary between brands (and field of use), so none of this is entirely standard', but roughly:

XWFL (Extra Wide Flood) 60-70° degrees,

VWFL (Very Wide Flood)

WFL (Wide Flood) 30-50°

FL (Flood)

NFL

MFL (Medium Flood) 20-35° (often elliptical)

SP (Spot) - perhaps the most ambigious one (perhaps 8 to 40 depending)

NSP (Narrow spot) 10-20°

VNSP (Very Narrow Spot) 5-15°

Note:

• The more typical stage-lighting ones are bolded, the angles seem to be the stage lighting ones

• e.g. most brands uses fewer of the above (often 4 or 5), and may have their own angle definitions, so the terms don't work very precisely between brands

• wider-throw bulbs will typically be used in shorter-nosed cans.

https://en.wikipedia.org/wiki/Parabolic_aluminized_reflector_light

Pinspot

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 (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.

https://en.wikipedia.org/wiki/Ellipsoidal_reflector_spotlight

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, balanced 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:

• https://en.wikipedia.org/wiki/Spotlight_(theatre_lighting)

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.

https://en.wikipedia.org/wiki/Scoop_(theater)

Fresnel lantern

A small(ish) 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

https://en.wikipedia.org/wiki/Fresnel_lantern

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

Unsorted

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.

Lens resolution measurement and correction

PSF, MTF, CTF

http://www.normankoren.com/Tutorials/MTF.html

https://en.wikipedia.org/wiki/Point_spread_function