The relationship of the brightness B and the anode voltage Va for the low energy electron luminescence(LEEL) may be expressed by

β

= QVanwhere Q and n are constants. This equation shows that the brightness is proprotional to Van. But at higher anode voltages

the brightness gees through a maximum and then decreases. That is so-called "brightness saturation" of LEEL. In this paper

the basic physical reasons of the brightness saturation have been discussed. It was pointed out that when Va=Vg(Vg is the grid voltage) the brightness saturation is not due to thermal quenching or dynamic saturation of the luminescent centres

but mainly due to the following processes:first

the electron emitting from the cathode is fatigued

resulting in a decrease of anode current density

and second

the ratio cf the anode and the grid current densities is decreased

since the secondary electron emission of ZnO:Zn increases with increasing applied anode voltage. For a flat triode luminescent device the ratio Ia/Ig may be given bywhere u is a constant

o(Va)is the effective cross section of the grid

d. and dg are the distances from the anode and from the grid

to the cathode respectively. When a voltage much less than that of the anode is applied to the grid

the secondary electrons emitted from the znO:zn will return to the anode

so the above expression can be simplified as As can be seen

the ratio of the anode and grid current densities increases with increasing (Va)

1/2

. In this case

the saturation brightness will obviously increase

e.g.

4230cd/m

2

for Va=Vg and 8918cd/m

2

for Vg

<

<

Va. It has been found that there is an optimal value of the anode current density

corresponding to the optimal luminescent efficiency. For a given input power

the luminescent efficiency at low anode current density and high anode voltage is always rather poor. As is well known

because the power efficiency of LEEL is only a few percent

most of the kinetic energy of the electrons is converted into heat. When the input power is high

the temperature of the luminescent layer will obviously increase. Therefore

when Vg

>

>

Va

the principal factor for the brightness saturation will be thermal quenching. The brightness is related to the input power W0 bywhere a and b are constants

B10 is the brightness at 10℃

P the power efficiency

the thermal conductivity

d the thickness of the anode substrate

and TO the temperature of the external surface of the substrate. For a given anode currect density

the optimal input power Wm corresponding to the efficiency maximum can be written asFinally

according to the results mentioned above

a luminescent device with saturation brightness of 37

000cd/m

2

as well as a high brightness vacuum fluorescent numerical display tube havebeen manufactured. The brightness of the display tube increases with increasing anode voltage and reaches a maximum(22

000cd/m

2

) at about 157V. At still higher voltages

the temperature exceeds the quenching temperature of the ZnO:Zn phosphor and the brightness decreases. The display tube can be steadily operated at the brightness of 5

000-10

000cd/m

2

(at Va=40-65V)

and a good legibility in a high ambient illumination (e.g.

bright sunlight falling directly or an ambient illumination of 100

000lux) can be achieved.