What are overhead projectors? They use transparent film slides and plastic sheets in A4 or bond paper size in order to display sketches, paintings, photocopies, printouts, and photographs placed on its flatbed component or platen.
When people think of projectors from the 2000s to the present, they usually think of digital video projectors they use for viewing movies or playing videogames on the big screen.
When people thought of projectors back in the 1990s and earlier, they either thought of slide projectors or overhead projectors used in school or at work. Overhead projectors or OHPs, in particular, used to be available all over the U.S.A. for business and education purposes.
With that in mind, what are overhead projectors? What should you consider when buying them in the 2020s or right now?
What are Overhead Projectors?
An overhead projector or OHP is like a slide or film projector. It uses light to project a huge image on a wall, screen, or similar surface. This allows a larger audience to view small documents, pictures, sketches, lyrics to a hymn, and so forth on the big screen.
The first known overhead projector was developed by Edmond Becquerel (a French physicist) in 1853. Jules Duboscq (a French instrument maker and photographer) demonstrated the capabilities of the new invention in 1866.
With the OHP, the image source doesn’t come from film strips or film slides. Instead, it uses a page-sized or A4 bond paper sheet of plastic or film which is also known as a transparency or foil. On this plastic sheet, the photograph, document, or image is drawn, written, painted, or photocopied.
How Do Overhead Projectors Work?
The light source below the clear sheet (the platen or the flatbed mirror light box reminiscent of the surface of a photocopier) and a projector mirror and lens assembly above it (hence the projector being “overhead”) allow the OHP it to project images.
OHPs were once widely used in schools, offices, conference rooms, and so forth before video projectors became more commonplace.
Why are Overhead Projectors not so Commonplace Nowadays?
OHPs were once the de facto projector type for use in U.S. classrooms, offices, conference rooms, meeting rooms, churches, art rooms, or any gathering where you had to look at an oversized version of an A4-sized document or picture along with a wide audience.
However, this all changed back in the 2000s, where they were replaced by interactive whiteboards, dedicated computer projection systems, and document cameras.
Overhead Projector Buying Guide
Should you still buy an OHP at this point? Perhaps as an alternative to a video projector, if it’s unavailable, you could. With it, you can paint, draw, or print images on an A4 plastic sheet or onion paper for display on the big screen.
The alternatives to OHP are numerous and mostly superior to it. Therefore, it’s a low-tech projector that has gone the way of the Dodo seemingly. However, if you insist on buying this type of projector, here are the things you need to keep in mind about it.
Scope out Your OHP’s Optical System
An OHP uses the same principles as the slide projector in order to work. It has a focusing lens that projects light from an illuminated slide care of a flatbed light box platen. A real image is formed onto the projection screen using the magnified contents of a transparency placed on the platen.
- Much Bigger Transparencies: The transparencies used on the OHP optical system uses much larger plastic sheets compared to the smaller photo film slides of a slide projector. The OHP transparency is typically the size of bond paper or a printed page, which is A4.
- Face-Up Presentation and Focusing Lens: The placement of the A4 transparency is face up to make it readable to the audience. The projection utilizes a mirror just before or after the focusing lens in order to fold the optical system towards the horizontal.
- Mirror Reversal: The mirror reversed the image so that the projected image corresponds to that on the transparency seen by the one doing the presentation. If you’re the presenter, you won’t have to flip the transparency over and you can read on the platen and the screen.
- Document Visibility on the Platen: The main appeal of the OHP optical system is that you can just look down and read the document or picture rather than needing to look at the screen to see what you’ve put up. A 35mm slide or film projector doesn’t offer such convenience.
A solar camera is a related projection and enlarging tech that also enlarges objects with a series of lights and mirrors. Meanwhile, the epidiascope uses the same projection technology as OHP but its optical system is closer to that of an opaque projector.
You may also like: What is a Slide Projector? Do They Still Make Slide Projectors?
The Fresnel Lens, Condenser, and Mirrors
When shopping for an OHP, pay attention to its condenser as well. Because the OHP’s focusing lens is smaller than the A4 transparency—it’s less than 4 inches or 10 centimeters on average—the optical condenser plays an important role when it comes to illuminating the transparency.
- The Fresnel Lens: The optical lens of the OHP should at least be the size of an A4 page. However, it could be of poor optical quality because the image sharpness doesn’t depend on it, so the Fresnel lens is usually utilized for the job. It redirects most of the light hitting it.
- A Lens That’s Located on the Platen: This lens is located on the platen or flatbed glass plate where the transparency is placed. The light it redirects goes into a converging cone towards the focusing lens. A condenser is required to target the focusing lens better.
- Why Condensers are Important: Condensers exist in order to ensure the light doesn’t miss the focusing lens. Condensers make OHPs cheaper because without it, focusing lenses would be exorbitantly expensive compared to the affordable Fresnel lens.
- Mirrors, Light Bulb Output, and Cooling Fans: Furthermore, the mirrors or other condensation elements below the Fresnel lens work to increase the output of the light bulb (or at least make it more efficient) before reaching the focusing lens. Also, the high intensity bulb often needs fan cooling in order to provide sufficient light brightness.
Long story short, a good OHP should have a fully functional condenser that assists in focusing the light of your lamp relative to the Fresnel lens. Check the specs for more details on condenser and mirror technology.
Overhead Projectors and Focusing Adjustment
A manual focusing mechanism is typically built into the OHP. They raise and lower the position of the focusing lens and its folding mirror for the sake off object distance adjustment. It makes the projection image on the screen clearer and more focused relative to the throw distance of the OHP.
It specifically adjusts the object distance or optical distance between the lens and the slide in order to get a better focus on a given throw distance or the distance between the projector and projection screen and the focusing lens’ focal length.
The manual adjustment mechanism allows you a wide range of projection distances the same way your video projector’s lens focus allows you to work with different throw distances. Obviously, an OHP has more limited throw distance options compared to its digital counterpart.
Why Overhead Projector Popularity Declined in the U.S.A.
Saved digital computer files and the systems that support them make it easier to display documents and pictures on the screen compared to the need for plastic sheets with words or pictures on them. The files are typically produced by LibreOffice or Microsoft PowerPoint.
- Expensiveness vs. Convenience: It’s relatively expensive to print or photocopy (Xerox) black & white or even colored transparencies for OHP use. A video projector’s digital files produces superior and convenient results without taking care of a ream of paper-sized plastic sheets.
- You Can Do More with Digital: When doing presentations on a video business projector, you can include things like video clips, interactive components, or animations on each of the paging between these virtual slides.
- Technology Marches On: OHPs became obsolete because modern society started depending more on computing technology and virtual files you can carry with you in floppy disks or the hard drives of PCs.
There were never any digital or PC-supported OHPs. Therefore, modern users who are used to using PCs for their business presentations or college thesis defenses went with digital video projectors for their needs instead.
Final Words
An OHP has the same tech available in opaque and slide projectors. It uses a series of lights and mirrors in order to display an image on the screen or wall. In this case, it uses transparent film or A4-sized plastic sheets instead of opaque objects or transparent film slides.
Just put the A4 clear sheet or onion paper on the flatbed surface of the OHP in order to display what’s drawn, printed, sketched, photographed or written there then adjust its size overhead. With that said, here’s our top OHP picks.
Worthwhile OHPs in 2021
The Apollo Overhead Projector is our best of the best pick. It offers sharp imagery, lasting use, and a high-efficiency cooling system in order to prevent overheating from its high-intensity light bulb with 2,000 lumen brightness. It costs $240. It measures 17.5 x 15.7 x 12.4 inches and weighs 13.23 pounds.
Meanwhile, the 3M 1700 Overhead Projector is our budget pick. This affordable transparency projector is ideal for PC-free, low-tech presentations using plastic transparency sheets. It measures 16.75 x 14.2 x 28 inches and weighs 18 pounds.
References:
- “Overhead Projector“, Wikipedia, Retrieved June 28, 2021
- “Art Projector Guide“, Art-is-Fun.com, Retrieved June 23, 2021