Phosphors
Phosphors
- Subject: Phosphors
- From: Robin Myers <email@hidden>
- Date: Fri, 08 Aug 2014 18:32:59 -0700
From: Robin Myers <email@hidden>
Subject: Phosphors
Date: August 8, 2014 at 6:31:58 PM PDT
To: Steve Jenkins <email@hidden>
> Oh, while we're on the subject, correct me if I'm wrong, but aren't the fluorescent agents, used for OB, absorbing UV, which we pretty much can't see, and the "agent" emits light at a lower frequency, to a part of the spectrum we do see -- which happens to be blue? Obviously there are phosphors out there that can fluoresce across the full spectrum(remember CRTs). So, is it the cost that prevents OBs from being full spectrum? Or some kind of energy issue ( i.e. CRTs used high energy electron beams to excite the phosphors)?
You are correct in how the optical brighteners (OB) or fluorescent whitening agents (FWA) in paper operate.
CRTs used three phosphors in checkerboard patterns or striped arrays to produce colors when activated by one (e.g. Trinitron) or three (traditional) electron beams. The blue and green phosphors were broadband, the red phosphor was usually one based on rare-earth elements and produced several narrow band emissions.
Most white LEDs are made with a single broadband yellow phosphor excited by a blue LED. This works well for lighting where the color rendition is not very important such as architectural lighting, street lighting, flashlights, etcetera. These LEDs are usually deficient in cyan and red emissions but seeing is more important than color quality with these LEDs.
Now that LEDs are starting to be used for retail purposes where the color is very important such as clothing displays, meat markets, cosmetics, etcetera, the LED manufacturers are beginning to use two or more phosphors to improve the color rendering properties.
On that same topic, fluorescent lamps use a similar method for creating visible light, just substitute UV from excited mercury vapor for the blue LED. Continuing that parallel there are fluorescent lamps with one, two or three phosphors. In the CIE list of standard fluorescent illuminants the lamps usually used for color tests are the F2 (single phosphor, cool white fluorescent), F7 (dual phosphor, D65 simulator) and F11 (triple phosphor, Philips TL84, Ultralume 40) lamps. More phosphors are ocassionally used to improve the color rendering such as in one viewing booth manufacturer's lamps where six phosphors are used to produce a high color quality light source.
I encourage everyone to learn more about light by experimentation. Some experimental supplies can be found at Edmund Scientifics (http://scientificsonline.com), Educational Innovations (http://educationalinnovations.com) or other experimental supply vendors. Try diffraction gratings to see the spectrum (some vendors offer glasses with diffraction grating lenses), or a cheap spectroscope (about $10) with a scale for getting a wavelength reading, then get out and measure the world. You might be surprised by what you will find.
Robin Myers
rmimaging.com
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