>=20
> On Jul 24, 2011, at 11:38 AM, Kerry Wentworth wrote:
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> Have you considered leaving the lights on and using a mechanical shutter?
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Neat idea .......
Apply silver paint to one side of tube and rotate the always-on tube.



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One could also "dim" the fluorescent tube......

Burst Dimming is a method of controlling the brightness of cold cathode flu=
orescent lamps (CCFL) by turning the lamps on and off at a rate faster than=
 the human eye can detect. The on/off rate is nominally 100Hz to 300Hz. The=
 higher the ratio of on-time to off-time, the brighter the lamps will be. B=
ecause of CCFL response times, on-time to off-time ratios of less than 1% a=
re not practical.



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http://www.maxim-ic.com/app-notes/index.mvp/id/4135
Abstract: The DS3881, DS3882, DS3988, DS3991, DS3992 and DS3994 are control=
lers for cold-cathode fluorescent lamps (CCFLs) that backlight liquid cryst=
al displays (LCDs). To achieve satisfactory visual results or conserve lamp=
 life, dimming is required in most applications. This application note firs=
t introduces the two dimming approaches commonly used for CCFLs. It then de=
scribes how to implement analog dimming on the  DS39xx CCFL controllers.=20

CCFL Dimming Methods

There are two common methods to dim CCFLs: burst dimming, which is also cal=
led PWM dimming or digital dimming, and analog dimming. This article discus=
ses the advantages and disadvantages of each method.

Burst dimming turns the CCFLs on and off at a certain frequency, which is c=
alled the PWM dimming frequency. If the PWM dimming frequency is greater th=
an 60Hz, the human eye cannot detect that the CCFLs are switching on and of=
f. During the high period of the PWM cycle, the CCFLs are turned on and wor=
k at the lamp frequency. During the low period of the PWM cycle, the CCFLs =
are turned off and no current flows through them. By adjusting the duty cyc=
le of the PWM pulses, one can thus increase or decrease the brightness of t=
he CCFLs. The principal advantage of burst dimming is that it can achieve a=
 very large dimming ratio. In some applications, however, the PWM dimming f=
requency may interfere with the vertical synchronous frequency of the displ=
ay signals, thereby causing visible effects on the screen. Burst dimming ca=
n also cause audible transformer noise.

Instead of turning on the CCFLs in bursts, analog dimming keeps the CCFLs o=
n continuously. Lamp brightness is adjusted by varying the lamp current amp=
litude. Obviously, a greater current amplitude results in a brighter CCFL, =
and a lower amplitude renders a dimmer CCFL. Analog dimming has a very narr=
ow dimming range, which is insufficient in some applications. But analog di=
mming does not cause any audible transformer  noise, since the PWM frequenc=
y is not present. Analog dimming, moreover, does not interfere with the ver=
tical synchronous frequency.


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