Re: PET 2001 phosphor - blue or white?

On Tue, Sep 13, 2016 at 1:03 PM, Anders Carlsson
<anders.carlsson@sfks.se> wrote:
> Hello,
>
> Every now and then, there is a discussion about how come so few computer
> monitors in the earlier generations had a blue display...
>
> White on black also seems to have been quite common, which leads me to my
> question:
>
> On Wikipedia, as well as the PET FAQ maintained by Larry Andersson, it is
> said that the first line of PET 2001 had a blue on black display. I always
> thought that was white on black, but if you bring down the brightness and
> have less lighting around the computer, it will glow and look light blue.

Blueish, yes, but not a solid blue, which is an available phosphor color.

> What does the monitor itself look like, can one based on which phosphorous
> used determine which hue it is supposed to be? If it just was Wikipedia +
> hundreds of pages quoting it that had got it wrong, I wouldn't mind but if
> one of our own FAQ's contain information that could be questioned, it makes
> me curious.

As mentioned elsewhere, there are two plastic frames around the 9"
(23cm) CRT in the original PET.  One frame is black, one frame is
blue, just to cover that aspect of black/blue in PETs.

The phosphor used, however, is quite standard and used in any number
of televisions and mono-displays of the era (DEC VT52, DEC VT100...)
The green P1 phosphor on PETs came in around the era they were
switching to dynamic RAM boards.  I have an original 8K static PET
with the blue plastic frame and at least oneUS PET 2001-N (European
3032) with the "white" P4 phosphor as well as several later 12" (31cm)
PETs with the green P1 phosphor, and I have seen a few 9" (23cm) P1
green PETs.

The typical color emissions are:
P1 - Green 490-580 nm
P3 - Yellow 504-700 nm (there are no amber PETs that I know of, but
one could swap out the tube...)
P4 - Paper White - 390-663 nm
P4 - Blue White - 326-704 nm

So even "white" is really blue plus another color, commonly yellow to
sum up to a white appearance, though there are "real" blue CRTs in the
420-430 nm range (more common in oscilloscopes).  It may be that
turning down the beam intensity excites the blue component in the
phosphor more than the yellow component, giving it a blue cast.

-ethan

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