Illustration adapted from Cameras by C.B. Neblette (c) 1952 (public domain)
Albada finder (bright-line finder)
The Albada viewfinder is named after its inventor, the Dutch military officer and optical designer Lieuwe Evert Willem van Albada. This device was first employed in the 1935 Zeiss Contaflex 35mm TLR. The rear face of the front lens of an Albada-type viewfinder is half-silvered, so that it reflects an image of a set of frame-lines painted on the surface of the eyepiece lens. The curvature of the rear face meets the need to reflect the frame-lines at the correct scale, and the front face is curved so that the scene also appears at the correct scale. The user sees a superimposed virtual image of the frame-lines at the same apparent distance as the image of the scene. The visible field normally extends beyond the frame-lines, making it easier to track moving subjects outside of the frame. The disadvantage of Albada-type finders is a tendency towards dim frame projection in low light.
The Albada finder is sometimes alternatively known as the "bright-line finder", which unfortunately invites confusion with the "bright-frame finder".
All Beauty 35mm cameras made before 1960 had Albada viewfinders: the easy identification clue to this is their silvered viewfinder objective lens.
Beam-splitter
This term is used in the descriptions "Bright-frame finder", "Parallax error and correction", and "Rangefinder". In general, a beam-splitter is an optical device that splits a beam of light in two. In these specific contexts, the beam-splitter apparatus is simply a piece of glass which is thinly mirrored, and therefore partially transparent. Some of the light hitting the beam-splitter is transmitted, while the remainder is reflected in a different direction. In a camera, the beam-splitter is effectively used in reverse to combine two sources of light into one beam (i.e. to overlay a reflected bright-line frame, and/or a rangefinder patch).
Bright-frame finder
The Bright frame finder was a method for presenting a sharply focused and illuminated sighting frame developed for military use in WWII. The innovation found its way into the 1947 Argus Model 21 where it was called the Markfinder. The bright frame finder has a half-silvered mirror (a beam-splitter) placed at an angle in the viewfinder line of sight, which combines the viewfinder image with projected frame-lines lit through their own light collecting window (often placed next to the viewfinder objective). The user sees a superimposed virtual image of the frame-lines at the same apparent distance as the image of the scene - just as in an Albada/Bright-line finder - but, in addition to providing a brightened view, the projected frame-lines also improve framing accuracy, since their apparent position is not affected by an off-the-axis user line of sight.
All Beauty 35mm cameras made from 1960 had bright-frame viewfinders. The easy identification clue is their additional illumination window: obstruct this while looking through the viewfinder, and the bright-frame disappears.
Illustration adapted from Cameras by C.B. Neblette (c) 1952 (public domain)
Light Values
A "Light Value" is a numeric representation of how bright a subject appears in absolute terms. It is a system for describing the luminosity of light, for example, 16 represents bright sun, while 10 is dreary and overcast. Its aim was to simplify choosing between equivalent camera exposure settings by replacing shutter speed and f-stop permutations with single number settings. This was regarded as especially helpful for beginners with limited understanding of the effects of different shutter speeds, and apertures, and the relationship between them.
Although not relevant to Beauty cameras, a later method for describing the luminosity of light is the Exposure Value, or EV system. This differs through the additional consideration of film speed and is therefore a measure of the light entering the camera, but it so happens that, with100 ASA film, the LV and EV value's meanings/descriptions are equal in all respects. This often leads to confusion between the two systems, and the presumption they are one and the same thing.
Parallax error & correction
Parallax error occurs when the image seen through the viewfinder is not framed exactly the same way as the image seen through the camera lens, because the viewfinder is in a slightly different position to the lens. This effect is most apparent with subjects close to the camera and becomes insignificant at longer distances. All cameras with a separate viewfinder and lens suffer from parallax error.
The simple method of correction for Albada/Bright-line viewfinders was the addition of supplementary viewfinder frame-lines, which indicated an alternative framing to be used at the camera's closest focus. Beauty 35mm cameras made between 1956 and 1959 (the 35 Super through to the Super L) all had these fixed horizontal viewfinder parallax correction marks.
The first Lightomatic seemingly (I haven't dismantled one - yet) had a mechanical parallax correction device, whereby the frame-line mask is physically shifted, via a linkage to the focus adjustment. The movement of a frame-line mask can be seen in this YouTube video($): although the demonstration camera is a Canon QL19.
The later Lightomatics (II, III and SP) had projected bright-frame lines. A mechanical linkage to the lens focus mechanism redirected the frame projection so that it moved synchronously with focus adjustments. A more comprehensive explanation can be found on the Lightomatic deconstruction page.
Lightomatic II viewfinder assembly. Highlighted in red, the bright-frame mask is lit by an opaque window, and projected via a mirror, lens and beam-splitter, and appears to hover around the viewfinder image. The mask hangs from a hinge, which allows the projection to be re-positioned via a linkage to the lens focus distance.
Rangefinder
An optical rangefinder is a distance-measuring device. The calculation of distance relies on the underlying mathematical principles of trigonometry, whereby unknown properties of a right-angle triangle can be derived from known properties.
An optical rangefinder combines images from two windows separated by a short distance. The two images are both viewed in the viewfinder eyepiece. The primary image is the entire viewfinder frame, while the secondary image is a superimposed, overlapping, small "patch" in the centre of the frame. To find the focus distance, the user looks through the viewfinder and aims the centre patch on the subject. Turning an adjustment dial causes the secondary image to move horizontally. At the point when the two images are perfectly aligned to become a single view (are "coincident"), correct focus is achieved.
In Beauty cameras (and many others) the rangefinder is a "coupled" coincident-image type. "Coupled" means the rangefinder operation is linked to the lens focus dial: it has no separate controls of its own.
The mechanics of a optical rangefinder rely on light entering a rangefinder window, and then being reflected by a mirror into a lens which focuses an image, along with a mask to trim and shape that image into a patch. This is then combined with the viewfinder image via a beam-splitter. Rotation of the mirror shifts the path of the patch, so it can be moved in and out of coincidence with the viewfinder image. A linkage to the camera lens focusing mechanism rotates the rangefinder image according to the distance at which the lens is focused.
More comprehensive details of the rangefinder mechanism can be found on the Lightomatic disassembly page.
Beauty cameras had either a pink or yellow, square, diamond, circular and then rectangular rangefinder patches.
Lightomatic II rangefinder. Light enters the rangefinder window, hits a mirror and is directed through a barrel with a focusing lens, then combined with the viewfinder image via a beam-splitter. The mirror rotates with focus adjustments, such that the rangefinder image is in coincidence with the focal distance of the camera lens.
Selenium light meters
When exposed to light, a Selenium photocell produces a tiny amount of electricity - proportionate to the brightness of the light (i.e. the brighter the light, the higher the current), The electrical current is typically connected to a pivoting electromagnetic coil with a lightweight needle attached. The needle travels across some form of scale that forms the basis of an exposure calculator. The light sensitive side of the photocell is covered by a lens; a honeycomb of small convex lenses which (in geek-speak) resemble "frogspawn". These narrow the field of light capture to roughly match that of the camera's objective lens. All light meter equipped Beauties had Selenium cells.
Selenium meters first became available to photographers in the early 1930s and had the advantage of "seeing" light in the same way as film does. Nevertheless, by the mid 1960s, Selenium meters were replaced by Cadmium Sulphide (CdS) cells, which were battery powered, light sensitive variable resistors. Despite the drawback that CdS "sees" light similarly to the human eye, and is therefore less compatible with film, they are more sensitive in very low light conditions and much smaller, making it possible for manufacturers to put the cell behind the lens in cameras. CdS ended the use of selenium.
There is a persistent myth that selenium cells "wear out" with age. This is partly true: their power-generation can diminish for a number of reasons. The action of UV light can damage a cell's sealing layer of varnish, which protects the selenium from oxidization. Factors such as exposure to humidity can corrode the electrical contacts to the cell. Otherwise, selenium cells - unlike a battery for example - do not necessarily deteriorate with age. In my experience, the chances of a selenium meter still working are good.