High-Pressure Mercury-Vapor Lamp Having a Plural Phosphor Coating

This invention relates to a high-pressure mercury-vapor lamp with a phosphor coating.

High-pressure mercury-vapor lamps are most popular as the lamps for outdoor illumination, and are expected to be widely used in the future because of their stable lamp characteristics and excellent economic advantages. These lamps may be roughly divided into the transparent type and the diffusion type. In the former, only the visible line spectrum of mercury generated from the arc discharge is utilized for illumination and accordingly, its color rendition index Ra runs as low as about 25; whereas in the latter, the ultraviolet radiations of mercury are utilized to stimulate the phosphors to cause them to emit visible rays by coating the inside surface of the outer bulb with a phosphor layer, to substantially improve its color rendering property. For example, these color corrected high pressure mercury vapor lamps in which such red light emitting phosphors as yttrium vanaphosphate or yttrium vanadate activated with trivalent europium are used, may provide a general color rendering index of about 42-45. Even these color corrected high pressure mercury vapor lamps having improved color rendition are still unsatisfactory for lamps for general indoor illuminations.

Lamps of high brightness for indoor illumination of offices and lobbies have recently come into great demand. High-pressure mercury-vapor lamps available on the market have failed to meet this demand, because of their inferior color rendering property.

The object of this invention is to provide high-pressure mercury-vapor lamps having greatly improved color rendition so that they are useful for general indoor illumination.

The present inventors conducted comprehensive and detailed studies including all kinds of lamps for the purpose of achieving this objective.

In the first place, they attempted to improve the color rendition of diffusion type lamps manufactured according to the same specification as those of mercury lamps available on the market, by coating the inside surface of the outer bulb with blue-green light emitting phosphors in addition to the red light emitting phosphor. As the blue-green phosphors, several types which had emission peaks at wavelengths of 440-540 nm were prepared. The lamps were manufactured, with these phosphorus mixed with red light emitting phospher in various mixing ratios, and their color rendition properties were measured. The lamps thus obtained had a general color rendering index of about 65, a value comparable to that of white light fluorescent lamps used for general indoor illumination purposes. Among these lamps having such high Ra values, however, some lamps were found objectionable from the standpoint of the visual sense, because human skin looks too yellowish when illuminated by them. In connection with this phenomenon, the large outputs of the mercury line spectrum at wavelengths 577-579 nm in the discharge tubes turned out to be relevant. The present inventors as a result of detailed studies, have discovered how to make high-pressure mercury-vapor lamps capable of good color rendition and that give favorable color rendition of human skin, by matching the energy output of phosphors with the output of the mercury line spectrum of the discharge tube in special relationships. The inventors examined the lamps, in regard to their color rendition in which the output from phosphors having emission peaks at wavelengths in the ranges of 400-540 nm and 600-720 nm, and the output of the mercury spectrum a wavelength of 546 nm and wavelengths of 577-579 nm were widely varied. As a result, they found that high-pressure mercury-vapor lamps not only provide general color rendition indicies higher than about 60, but also give a favorable color rendition human skin, provided the following three relationships are satisfied simultaneously.

where

P.sub.577-579 : mercury line spectrum outputs at wavelengths of 577-579 nm.

P.sub.546 : mercury line spectrum outputs at 546 nm.

P.sub.440-540 : total emission output in the range of wavelengths of 440-540 nm

P.sub.600-720 : total emission output in the range of wavelengths of 600-720 nm

(Note: The units used all through this specification are relative values.)

(Provided that P.sub.440-540 and P.sub.600-720 all do not include the output of the mercury line spectrum.)

The inventors also found that the same results can be obtained if, instead of the above formula (1), the following formula (1') can be satisfied on the mean electric potential gradient Ed(V/cm) of the arc in the discharge tube (Ed = (lamp voltage)/(distance between the electrodes)).

FIG. 1 is a partially broken away sectional view showing a high-pressure mercury-vapor lamp embodying this invention;

FIG. 2 is a graph of light transmissivity of a neodynium glass as a function of wave length.

FIG. 3 is a graph showing the P.sub.577-579 / P.sub.546 ratio as a function of between electrode distance.

FIG. 4 is a graph of the color rendition index as a function of P.sub.577-579 /P.sub.546 ratio.

FIG. 5 is a graph showing the ratio of various response of persons to color rendition of their skin as a function of the P.sub.577-579 /P.sub.546 ratio.

FIG. 6 is a graph of a general color rendition index as a function of light transmissivity of the outer bulb.

FIG. 7 is a graph showing the ratio of various responses of persons to color rendition of their skin as a function of the P.sub.440-540 / P.sub.577-579 ratio.

FIG. 8 is a graph of the general color rendition index Ra as a function of the P.sub.440-540 / P.sub.600-720 ratio.

FIG. 9 is a graph showing the ratio of various responses of persons to color rendition of their skin as a function of the P.sub.440-540 / P.sub.577-579 ratio.

FIG. 10 is a graph showing the ratio of various responses of persons to color rendition of their skin for various lamps.

Referring to FIG. 1, numeral 1 represents a discharge tube made of a transparent material, for example quartz, to both ends of which electrodes 2 and 3 made of tungsten, and auxiliary electrodes 4 and 5 are sealed respectively with intermediate molybdenum foils 6, 7, 8 and 9 for the respective electrodes. Inside the discharge tube, an appropriate amount of mercury (which will be described later), and 30 Torr of argon for starting-up are sealed in. The discharge tube is supported by supporting plates 10 and 11 on stem wires 12 and 13, the conductors 14 and 15 coming from the electrodes 2 and 3 respectively being electrically connected to the stem wires 12 and 13, and the conductors 16 and 17 from the auxiliary electrodes 4 and 5 respectively being electrically connected to the stem wires 13 and 12 respectively through the starting resistances 18 and 19. The stem wires 12 and 13 are associated with the base 22 through the lead wires 20 and 21. The discharge tube is further incorporated within and is supported by the outer bulb 23 made of hard glass, in which an inert such as nitrogen is sealed. On the inside surface of the outer bulb 23, a phosphor layer 24 is applied.

The various phosphors used have emission peaks at wavelengths 450 nm, 490 nm, 505 nm, 518 nm or 617 nm respectively, as shown in Table 1. Such phosphors were applied on the inside surface of the outer bulb either singly or in combination of more than two.

To begin with, lamps with various P.sub.577-579 / P.sub.546 ratios were prepared and their color rendition was examined. The ratio P.sub.577-579 / P.sub.546 can be varied, by using an outer bulb containing neodymium, by coating the outer bulb with neodymium oxide mixed with phosphor powder (FIG. 2), or by varying the distance between the electrodes (FIG. 3). The outer bulb was coated with phosphor material which has a mixture of phosphers C, D and F of Table 1, in a weight ratio of C : D : F = 2.2 : 0.8 : 0.8. Lamps thus prepared were lit with wall load of 7-18 W/cm.sup.2, and the color rendering property was examined, and the following conclusions were reached;

1. The general color rendition index Ra and the special color rendition index R.sub.15 increase as the ratio P.sub.577-579 / P.sub.546 decreases, as shown by FIG. 4.

2. Color rendition of human skin is greatly improved, provided the ratio P.sub.577-579 / P.sub.546 is in the range 0.50-1.0, especially from 0.60-0.90, whereas lamps with a P.sub.577-579 / P.sub.546 ratio larger than 1.0 give a light which is too greenish, and lamps with a P.sub.577-579 / P.sub.546 ratio less than 0.5 give a rendition which is too greenish as shown in FIG. 5.

Lamps with the P.sub.577-579 / P.sub.546 ratio in the favorable range of 0.50 to 1.0 can be obtained by making the potential gradient of the mercury arc Ed between 21 V/cm and 52 V/cm.

In the case of 200W, 300W and 400W lamps, the lamp voltage of which must be 120 to 140V, an arc length of 2.5 cm to 6.2 cm is preferred.

Next, favourable spectral distribution was sought for by various combinations of the P.sub.577-579 / P.sub.546 ratio and phosphor materials with various emission spectral bands and the following conclusions were reached;

1. As long as the ratio P.sub.577-579 / P.sub.546 is in the range 0.50- 1.0, irrespective of the phosphor material, lamps give a favourable color rendition of human skin.

2. Generally, by reducing the output of the line spectrum by curtailing the transparency of the outer bulb, and by increasing the output of the band spectrum from the phosphor, both of which effects are achieved by the application of a thick layer of phosphors, the general color rendition index Ra increases as shown in FIG. 6. If the radiation between wavelengths of 440 and 540 nm is too strong, human skin is rendered displeasingly blueish or greenish, although the yellowish effect of wavelengths of 577-579 nm is supressed and the Ra value is rather high.

This can be remedied by controlling the ratio P.sub.440-540 / P.sub.577-579. From the color rendition study, the results of which are as shown in FIG. 7, the ratio P.sub.440-540 / P.sub.577-579 is preferably in the following range,

specifically, it is preferred that the ratio be in the range of 0.4-0.9. If the ratio is less than 0.3, the rendition is too yellowish, and if the ratio is larger than 1.2, it is too blueish or greenish.

3. Radiation by phosphor materials was divided into two parts; radiation between wavelengths 440 and 540 nm, and radiation between wavelengths 600 and 720 nm, and the effect of the ratio P.sub.440-540 / P.sub.600-720 upon color rendition was investigated.

As a result, Ra was found to be most favourable when this ratio is between 1.1 and 1.4, as shown in FIG. 8. The lamps with the highest value of Ra, however, gave unpleasant results to observers due to lack of a red component. In order to render the color of human skin well, the ratio P.sub.440-540 / P.sub.600-720 is, irrespective of the Ra value, preferably in the range 0.1 to 1.0, especially 0.2 to 0.8, as shown in FIG. 9.

4. According to another opinion, it is a sine qua non for fluorescent light having a blue color output to have its peak at wavelengths of 440-470 nm to obtain a desirable color rendition of the skin. However, when the present inventors manufactured lamps with a mixture of phosphors having their peaks at wavelengths 450 nm, 490 nm, 506 nm or 515 nm, as shown in Table 2, and when the inventors made comparative observations of the color rendition effect of each of these lamps on the skin in color rendition tests, the test results as shown in FIG. 8 revealed no superiority of the phosphors having a peak at wavelength of 450 nm. The reason is because the lamps of this invention have discharge tubes of such a design that the adverse effect on the skin of the line spectra at wavelengths of 577-579 nm is already inhibited. The results of calculations made by the present inventors by way of simulation, by making use of an electronic computer showed that the highest value of the general color rendition index Ra was obtained by using phosphors having peaks at wavelengths of 470-530 nm. This contradiction may be partially recognized from the results of tests as shown in FIG. 10. Furthermore, it is desirable that phosphors having a 450 nm peak be employed in a mixture with those having peaks at a wavelength around 500 nm.