Not to argue but for years Van Keuren was he de facto standard for optical flats and machine shop gaging. Every shop posessiong a lapping machine had a VK monochromatic light and a scratched up flat neatby. Its light was neon pink corresponding to 630 nm (6300 Angstroms then). Given 630 nm another defacto standard sprang up whereby a single interferance fringe represented 1/2 wavelength of light or 11.4 microinches (I think. I gotta look it up.)
If green is the light freq to use then that's a new one on this old dinosaur. Next I suppose you'll say horses are out.
Hi Forrest,
That's correct. I was just looking through the 1948 Van Keuren catalog/handbook, in which they introduce that light source. It mentions that the company pioneered optical precision measurement in 1920.
The red/orange monochromatic source uses a helium-filled discharge tube operating at high voltage (3000 volts). This is one of the few natural sources of monochromatic visible light. I believe they still offer a source of this type. That is implied by the photo on their website, but they give no specs on the source.
Another commonly-available source was the high-pressure mercury vapor lamp. These produce good intensity, but require heavy filtration to achieve monochromaticity, and require substantial power, thus generating heat. And they're expensive. I don't know what color was used for measurement, since I've never seen a living example of this product, nor a spec sheet.
I don't know where this fits chronologically WRT the helium discharge tube. I suspect mercury was the first available source. Mercury lamps were in common use for industrial illumination in the 1920's.
The third version used small fluorescent bulbs of the desk lamp variety, coupled with a precision green monochromatic filter, since these lamps produce almost no energy in the red portion of the spectrum. These became quite popular due to low cost, simplicity, low power consumption, and very low heat. The Edmund product which I linked above is typical. Green has the added advantage of producing higher accuracy, as I mentioned previously.
The current state-of-the-art uses LEDs. I'm sure this will become predominant in the near future just because LEDs are overtaking all other technologies as the light source of choice.
But rest assured that horses are still very popular.
Thanks.
- Leigh