The work function of an oxide-coated tungsten filament in low pressure Hg discharge lamps was measured using the photoelectric effect with and without discharge. The plasma had been excited in a lamp with 2.45-GHz microwaves to reduce the power loading to the electrodes, which made the measurement of photoelectric current from the electrodes possible in plasma. Lasers with wavelengths of 325, 405, 532, and 633 nm irradiated the surface of the electrodes to induce photoelectric current. The work function of the electrode of an as-received lamp was determined to be 2.5 eV. Discharge operation of the lamp reduced the work function by activating the oxide-coated materials. The lowest observed work function ranged from 2.2 to 2.3 eV.
Characteristics of thermionic emission from emissive oxides were investigated by using experimental data of work function measurement of commercial fluorescent lamp electrodes presented in our previous paper. Our aims are as follows: first, to reconfirm the difference in the expression of equations describing thermionic emission current density between oxides (Wilson and Nijiboer models) and metals (the Richardson-Dushman equation). Second, we wish to further clarify the relationship between the temperature dependence of work function and the effective value of the Richardson constant.
Discharge lamps that have electrodes are classified into two categories, hot cathode lamps (HCLs) and cold cathode lamps (CCLs). We investigated the relationship between the presence of Hg, the kinds of rare gases present, and the cathode fall voltage (CFV) of both CCLs and HCLs. The results show mainly two points. One is the difference in the CFV characteristics, that is, CCLs’ CFV is positive, while HCLs’ is negative. This is because the emission mechanism of CCLs, due to the gamma effect, differs from that of HCLs due to the Schottky effect. The other main point is the influence of Hg on the CFV. The CFV of CCLs increases when Hg is present because of the difference between γ-coefficients for Ne+ and Hg2+ due to the energy potentials. The CFV of HCL also increases when Hg is present because of the difference between the cathode temperatures in an Ar discharge and an Ar-Hg discharge.
The secondary electron emission properties of MgO for liquid crystal display backlighting electrodes were investigated. The breakdown voltages for MgO electrodes were measured under various Ne pressures by the V-Q Lissajous method. To measure the breakdown voltages of MgO, the samples tested were MgO single crystal plates with polished surface and non-polished surface and MgO sintered ceramic plates. The breakdown voltage for MgO single crystal plates with non-polished surface was lower than that with polished surface. The breakdown voltage for MgO sintered ceramic plates was slightly lower than that for MgO single crystal plates with non-polished surface. The secondary electron emission properties were affected by the surface roughness of the MgO plate.
Equivalent luminance contrast was investigated as an index that expresses the relationship between color difference and readability of chromatic documents. The relationships between the equivalent luminance contrast and the visual factors were determined. The visual factors are background luminance (which represents adaptation luminance in the visual field with uniform luminance), visual angle of the letters, luminance contrast, and color difference between the letters and the background. For chromatic documents, equivalent luminance contrast was found to be affected by background luminance and the visual angle of the letters. With regard to the various color combinations of background and letters, the relationship between the color difference and equivalent luminance contrast asone of the visual factors was determined. Equivalent luminance contrast was high with a large color difference. Even when there is no color difference, equivalent luminance contrast depended on the direction of the letter color from background color in the CIE1976L*a*b* color space. This directional property was almost the same for all background colors.