In this report we have clarified that (1) pH of PVA-phosphor slurry and UV light intensity are correlated with the penetration depth of the incident UV light into dried PVA-phosphor screen; (2) particle surface

particle shape

particle size distribution etc.are correlated with scattering of incident UV light in a dried PVA-phosphor screen;(3) mean particle sizes determine a screen weight to give an optimal screen thickness;(4) a profile of the UV light intensity

exposure dose and development conditions are predominantly correlated with sharpness of phosphor dots or stripes.In aqueous solution amonium dichromate(AD)

(NH

4

)

2

Cr

2

O

7

hydrolyzes and several species of chromete ions:Cr

2

O

7

-2

HCrO

4

-2

and CrO

4

-2

exist in different ratious at different pH value and AD concentrations. But only HCrO

4

-2

ions respond on photosensitization of PVA. Fig. 1 shows the absorption spectra of aqueous solution at two pH values but the same AD concentration 2×10

-4

mole per liter.The dependence of the HCrO

4

-1

concentration on pH value is shown in Fig. 2. The pH value must not be smaller than 7. otherwise the incident UV fight Will be totally absorbed by a thin surface layer of PVA film and no light will reach the substrate. Consequently the phosphor particles do not adhere on the substrate.Empirically it is shown that a simple maximizaton of the concentration of HCrO

4

-1

is not reality in screening of phosphor powder even if photoreaction of PVA(AD) resin is essential.Hence we must cosider the optical properties of PVA-phosphor screen with fhe assumption of small absorption coefficient of PVA(.AD) resin.Since the average size of phosphor particles is around 3 to 4um and the absorption coefficient of phosphor to the UV light is estimated to be 10

5

per cm (0.1μm depth)

the UV light never penetrates through the particles. The photoreaction of PVA (AD) resin at substrate occurs mainly due to the absorption of scattered UV light by phosphor particles.The scattering in phosphor srecen increases as optical irregularity of phosphor is increased. And after baking process the dried adhesion of particles decrease as particle size is increased.Te minimum size required meets also the requirement of high optimum brightness

approximately 3 to 4um. Maximum scattering is expected with round particles.Further the distribution of particle size should be according to log-normal probability.This says that the particles are primary (i.e.no aggregated particles). Fig.6 illustrates a tool for study of screening conditions.The screen is dried after screening of PVA-phosphor slurry and is exposed by the UV light through an attenuator filter which changes its transmittance continuously from 100% to 20%.Adhesion of phosphor particles is determined from the threshold angles at which the screen defects start after development.Then we discussed the problems of optimum screening Weight per cm

2

profile of UV light intensities on phosphor screen and cross contamination.