What Does an LE-500 Rating Mean for Archival Microfilm?
Share
An LE-500 rating (often written LE500) means archival microfilm is expected to remain usable for 500 years. The rating holds only when three conditions are all met: the film meets ISO 18901, it is processed to ISO 18917 limits, and it is stored under ISO 18911 extended-term conditions. LE stands for "life expectancy". It is the rating national archives and records managers specify for preservation masters.
The three conditions behind LE-500
- The film (ISO 18901). A silver-gelatin emulsion on a polyester base that meets the stability specification. Acetate-base film does not qualify.
- The processing (ISO 18917). Fixer residue (residual thiosulphate) must stay below set limits, or the image will degrade over time. This is checked with the methylene blue test.
- The storage (ISO 18911). A cool, stable temperature, controlled relative humidity and clean air, in archival enclosures.
If any one condition fails, the rating no longer applies. A roll of LE-500 film that is poorly washed or stored in a hot, humid room will not last 500 years.
What LE-500 does not mean
- It is not a property of the film alone. It is a rating for film, processing and storage together.
- It does not apply automatically to duplicates. Use copies made on other film types carry their own ratings.
- It is not a guarantee of image quality. Legibility is a separate test, against ISO 6199 resolution targets.
How to specify LE-500 in a tender
- Ask for the film's specification sheet stating LE-500 to ISO 18901.
- Require methylene blue test results to ISO 18917 for each processing batch.
- State the storage conditions to ISO 18911, or confirm the vault meets them.
- Ask for resolution and density readings on each roll.
LE-500 microfilm from Micrographics Data
Micrographics Data, a Singapore microfilm supplier since 1989, stocks LE-500 rated silver-halide film: 16MGD-HR 16 mm archival microfilm rolls, 35 mm archival microfilm rolls and MGD33 archive-writer film. All pass the ISO 18917 methylene blue test when processed with matched microfilm chemistry. To write digital files to this film, see the computer output microfilm (COM) guide.
Frequently asked questions
What does LE stand for on microfilm?
LE stands for life expectancy. LE-500 is a predicted usable life of 500 years under the conditions set by ISO 18901, ISO 18917 and ISO 18911.
Is LE500 the same as LE-500?
Yes. Both spellings refer to the same 500-year life-expectancy rating.
Which standard defines the LE-500 rating?
ISO 18901 is the stability specification for processed silver-gelatin film. The rating also depends on processing to ISO 18917 and storage to ISO 18911.
How is processing quality checked?
With the methylene blue test in ISO 18917, which measures residual thiosulphate left in the film after washing.
Does digital storage have an equivalent rating?
No. Digital media have no comparable life-expectancy rating and need regular migration to new hardware and formats. Many archives keep digital files for access and LE-500 microfilm for preservation.