| Simply
stated, an overhead projector displays images to an audience.
Overhead projectors typically consist of a large box containing
a very bright light-source, a fan for cooling the box, and
a large lens that focuses the light. Above the box, typically
on a long and folding arm, is a mirror that redirects the
light forward instead of up. Page sized sheets of transparent
film are placed on top of the lens for display.
The light from the lamp travels though the
transparency and into the mirror where it is reflected onto
a viewing screen. The mirror allows both the presenter and
the audience to see the image at the same time, with the presenter
looking down as if writing and the audience viewing the screen.
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Overhead projectors were once a common fixture
in most classrooms and business meetings. Increasingly overhead
projectors are being replaced by larger computer monitors
or LCD projectors. Critics feel that these newer systems are
more expensive, much more prone to failure and have a steeper
learning curve than the traditional overhead projector. As
a result the overhead projector continues to be a popular
presentation accessory. The History of Overhead Projectors
The first projector used as a presentation
device was not the overhead but the opaque projector. The
opaque projector displays non-transparent materials by shining
a bright lamp onto the object from above. A system of mirrors,
prisms and/or lenses is used to focus an image of the subject
onto a viewing screen.
Opaque projectors require brighter lamps and
larger lenses than overhead projectors. Moreover, care must
be taken that the materials are not damaged by the heat generated
by the light source. Suitable objects for display and projection
include book pages, drawings, mineral specimens, leaves, etc.
In the early and mid 20th century, opaque projectors were
produced as low cost toys for children.
They were also marketed as an artist's enlargement
tool to allow images to be transferred to a prepared canvas.
Eventually they came into widespread use for lectures and
presentations. In its modern incarnation the Mark IV Opus
Opaque Projector by Buhl still has a loyal following. For
the most part, however, there is greater demand for overhead
projectors than opaque projectors in the modern market. Features
of Overhead Projectors
All overhead projectors share standard features
which should be considered as the basis for your purchasing
decision. The most important are lens type, brightness (lumens),
lamp life and of course, price. Let's consider how these features
can be measured and compared, as well as one additional variable
which I will call innovation. * Lens Type:The most significant
factor in overhead projection image quality is the type of
lens being used.
We offer projectors that feature the three
most common lens types: singlet, doublet, and triplet. Image
quality increases as you progress from the basic singlet lens
type, to the advanced triplet lens type, though price generally
increases, as well. * Brightness (Lumens):Each overhead projector
is rated on brightness by "lumens typical." For
rooms where the ambient light can be lowered, the standard
2000 lumens will suffice.
When you are projecting in a room where ambient
light is significant or when projecting complex images (including
color), 2,500 lumens or greater is recommended. * Lamp Life:Each
overhead projector has an estimated lamp life that is primarily
determined by the rating in lumens and lamp type. The common
ENX lamp type, for example, offers the longest lamp life while
the higher lumens and more specialized nature of other lamp
types (FXL, EHA, ENG) offer shorter lamp lives.
In the case of Buhl, we have found that some
newer and more innovative features have given them them a
decisive edge over 3M and other manufacturers. * Innovation:As
an industry pioneer, 3M has been responsible for many of the
major advances in projector technology. In recent years, Buhl
has siezed the initiative and successfully innovated new features
like 360 degree rotating heads, double fresnel lenses, rack
and pinion focusing, ambidextrous operation, auto lamp changers,
andHi-Lo switching.
As some of these innovations are patented
Buhl has gained an advantage in innovation over 3M. Overhead
Projector Configurations
Overhead Projectors come in many configurations
which lend themselves to different performance and cost criteria.
The majority of the applications are for displaying images
in rooms where the distance from the projector to the screen
is 6-12 feet. This is referred to as the projection distance
or throw distance. Projectors are made that go beyond the
range noted but are specialized and typically custom made
products.
For the conventional projection distances,
an overhead projector can be found in the following optical
designs: 1 - Direct Optics Configuration - in this Overhead
Projector format the light source and the associated optics
are positioned on a straight line known as an optical axis.
The light source is mounted in a lamp housing in conjunction
with a spherical reflector located immediately under the optical
center point of the Fresnel condensing lens.
These components are housed in a metal or
plastic body. The remaining optical component "Projection
head" is mounted of a post and arm assembly. The Projection
head has at least two elements, objective lens and a front
surface mirror. This assembly is also centered on the optical
axis and the mirror can be articulated to direct the image
from the projector stage to the screen.
The whole head assembly is moved along the
optical axis via a mechanical adjustment to allow the images
to be focused. The lamps for the direct optics are usually
two pin prefocused Halogen lamps. Wattages can range from
250 watts to 600 watts for conventional units. The package
for this type of Overhead Projector is the most common and
offers the lowest cost product to the end user.
2- Folded Optics - Also know as "Chamber
Optics" is a method of interposing a front surface mirror
between the Light source and the Fresnel lens. The focal length
of the objective side of the Fresnel Lens (the side facing
the lamp) is increased for this purpose. The light source
is typically a lamp mounted into a Elliptical faceted reflector.
This design combination yields a number of benefits when compared
to direct optics: - Superior light collection - more light
being used to project .
On average a minimum increase of 50% compared
to equivalently powered direct optics units. - Improved resolution.
The longer back focal length of the fennel bends the light
less and reduces optical aberration. - Stylish housing design
- generally more ergonomic and user friendly for use and maintenance.
- Improved cooling- using a reflectorized lamp allows the
use of thermal transfer coating on the reflector known as
a Dichroic Coating.
The coating allows the thermal output of the
lamp to be channeled through the reflector to the rear of
the lamp. This increases the efficiency of removing heat from
the Overhead Projector and dramatically reduces the stage
temperature when a transparency is being projected. The benefits
are countered slightly by a higher cost because of the extra
components and specialized lamp.
3 - Reflected Optics - The two previous designs
are typically table top models. Reflected optics is a configuration
expressly for the purpose of portability. In this design the
light source, cooling device and head projection optics are
housed in a single unit positioned above a projection stage
via a supporting arm or arms. This assembly has a mechanical
release device allowing the entire unit to break down into
a slim profile on the order of an attaché case.
The key to this design is in the fresnel lens
specialized for this optical system. The fresnel is a single
condenser backed by a mirror. This allows the light source
to direct light to it at which point light passes through
the lens gets reflected by the mirror. The reflected light
then passes back through the lens were it is projected to
the head objective lens.
Light gets focused through the objective
lens to a second articulating mirror directing the image to
the screen. The significant benefit is the portable package
allowing easy transport for presentations anywhere. The design
make sacrifices in performance for this benefit in there is:
- Reduced light output - light has to be projected
from the head to the stage where the transparency is positioned.
Light has to pass through the transparency twice that doubles
the loss of transmitted light though the media. The reflected
design has one additional mirror in comparison to the folded
optical design creating addition reflecting light losses.
- Double imaging - since light passes through
the media and off the mirror under the lens the transparency
must be as flat as possible to prevent a double image being
projected.
- Resolution - although the optics employed
for this design are of a higher quality and cost the nature
of the design has a reduced resolution. This is because the
alignment of the optical elements has to be precise in order
to get an acceptable projection. Tolerance variations in the
optical alignment and the quality of the transparency used
impact the overall image quality.
The true benefit of this design is the portability
but because of the precise optics particularly the specialized
fresnel and the mechanics of the package this is one of the
more expensive versions of Overhead Projectors.
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