From the 1950’s through the 1980’s, photocopiers created a copy of text or an image by reflecting light off of the white areas of the original (which had been laid face down on the glass platen) onto a charged drum. This reflected light neutralized the charge on the drum so that when oppositely charged toner was applied to the unexposed (and still charged) areas of the drum it would stick. The toner was then transferred from the drum to the paper and fused by heat and pressure to create the final copy. Laser printing is an updated version of this process. Now the original document or image is scanned and an electronic file is created. The data in this file is then used to control a laser which exposes the charged drum. From there, the rest of the process is the same with the toner being transferred to the paper and then fused to make it permanent. The process, whether to make direct photocopies from originals or laser prints, is technically known as electrophotography.
While the final copy (toner on paper) is basically the same now as it was 50 years ago, the path to print has clearly changed. Because a laser can be controlled by any electronic data set, the laser printer can be used not only for scanned copies but for original files as well. So the laser printer/copier can be used for both copying and printing. In fact, the photocopiers that populate the world’s offices today are really laser printers coupled with scanners.
In general, text printed with either pre-laser photocopiers or modern laser printer/copiers will look roughly the same, though the line quality of the text has certainly improved over time. Digitally printed images, whether from a file or copied from an existing photograph, will show regular dot patterns when printed with a laser printer (because it converts the image to a half-tone –see figures 2 and 3), but have random or no dots when printed with a pre-laser photocopier (see figure 1) unless the copied image was half-tone to begin with.
| Figure 1. Traditional B&W electrophotographic print (no regular dot pattern) | |
|
|
|
|---|---|
| Pre-laser Photocopy | Micrograph of toner particles |
| Figure 2. Modern B&W electrophotographic print (laser dot pattern) | |
|
|
|
|---|---|
| Laser-printed Image | Micrograph of toner particles |
| Figure 3. Modern color electrophotographic print (laser dot pattern) | |
|
|
|
|---|---|
| Laser-printed Image | Micrograph of toner particles |
In terms of preservation though, it matters little whether the print was created with a pre-laser photocopier or a modern laser printer, the primary determining factor is usually the paper. The colorant, however, can still be sensitive to decay. Preservation practices remain the same. Data from the Canadian Conservation Institute, presented in their Technical Bulletin No. 22 in the article The Stability of Photocopied and Laser-printed Documents and Images: General Guidelines by David Grattan, showed that the primary determinant of electrophotographic print longevity is the quality of the paper. IPI data has verified this including for the new electrophotographic digital presses. It should be noted that most office printer/copier papers were acidic prior to the 1990’s. However, those since have been primarily alkaline. Prints on recycled paper or other unstable paper may need cool or cold storage.
For more information on the electrophotographic process visit the technologies section [2].