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CHAPTER XII.. The Physiological Light Unit.

Light and Colour Theories, and Their Relation to Light and Colour Standardization · Joseph W. Lovibond — chapter 13 of 16 · ~2,383 words · public domain

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The Physiological Light Unit.

DIFFUSED LIGHT.

The physiological values of light intensities determined by the absorptive method, differ in some respects, from the intensities based on the inverse ratio of the squares of distance between the shadows of two lights.

However valuable this method of calculating light intensities may be from a mathematical point of view, it does not express the physiological appreciation of light differences.

The dimensions of the light unit used in the following experiments have already been described.

This method cannot deal exhaustively with intense direct lights, on account of the presence (activity) of the disturbing red ray which prevents total absorption. Such lights must first be modified by intercepting media of known diffusive value, or by reflection from a white surface.

One object of these experiments was to obtain more insight into the physiological conditions of light, as bearing on the question of standardization.

The first two experiments are records of intensity changes by time, in Morning and Evening light, and are of interest, as bearing on the lowest luminosities for reading, for viewing objects at different distances, and for defining the limits at which colours visually disappear. The measurements are marked in neutral tint units and plotted in curves in Charts 1 and 2.

The numbers on the single curve in the Morning chart represent total absorption of the direct light 20 degrees above the horizon. The upper curve in the Evening chart also represents the direct light, whilst the under curve represents the light values as reflected from a lime sulphate surface; except in the case of the reading notes when it represents the printed paper surface. The difference between the two curves is the loss of light incident on reflection, but this must not be rigidly interpreted for all cases, as there is reason for supposing it varies with different lights.

DIRECT LIGHTS.

In measuring the physiological intensity of direct lights, the presence (activity) of the unabsorbable red ray, prevents their being dealt with directly by the absorptive method. Such lights can, however, be made measurable by a sufficient diffusion, as already explained in the case of direct sunlight, the proportion of diffusion required, being more or less according to the intensity of the light; in the following examples, one reflection from a white surface or from the ordinary grease spot arrangement, was found to be sufficient.

Note.--Light reflected from grease is not above suspicion, it being governed by the law of specific absorption already dealt with.

Evening Light March 18^{th}. 1906.

Upper Curve 20° above Horizon Lower Curve as Reflected from a 1 Foot Square Lime Sulphate Screen ]

Morning Light Dec^r. 17^{th}. 1907.

20° above Horizon]

Luminous Intensities of 1, 2 & 3 Standard Candles.]

Luminosity of Gas from Burner

Reduced to Equal 1, 2 & 3 Candles at 1 Foot.]

Luminosities of Paraffin Lamp

Reduced to 1, 2 & 3 Candles at 1 Foot.]

The experiments were carried out on a home-made twelve-foot photometer, with the usual protected lantern at each end, one being removed for present purposes. The grease spot carrier was replaced by a one-foot square diaphragm, with a standard white surface; this travelled the whole length of the photometer at right angles to the light, and the readings made at one-foot intervals at an angle of 45 degrees, 10 inches distance from the white surface.

Charts 3, 4 and 5 deal with experiments, with one, two and three standard candles of the London Gas Referees. The curves represent the physiological rate of declining luminosities by distance. Some characteristic differences from other lights are brought together in Charts 4 and 5. The slight irregularities in the curves are probably due to the readings being made at half unit intervals. These acting sometimes in opposite directions, fully account for want of symmetry in the curves.

A noticeable feature in these experiments is the small amount of physiological luminosity added by each successive candle to the first. Theoretically, if one candle equals 21 units intensity, two should equal 42, and three 63, whereas the physiological additions of luminosity are not only much less, but vary with different luminous intensities, as will be seen by the following comparisons:--

1 2 3 Candle Candles Candles Total.

Theoretical 21 + 21 + 21 = 63

Physiological 21 + 3 + 6 = 30

Chart 4 represents the Luminosity of gas from a batswing burner reduced to one, two and three candle power at one foot distance.

1 2 3 Candle Candles Candles Total.

Theoretical 21 + 21 + 21 = 63

Physiological 21 + 3 + 7 = 31

Chart 5 represents the Luminosity of a hand Paraffin Lamp similarly reduced.

1 2 3 Candle Candles Candles Total.

Theoretical 21 + 21 + 21 = 63

Physiological 21 + 3 + 6 = 30

PHOTOGRAPHIC ENERGIES OF DIFFERENT RAYS

The following experiments are preliminary only, and were undertaken to determine the relationship of the several colours of the tintometrical scales, to their associated photographic values, under different conditions of light and times of exposure, the end in view being the hope of standardizing screens, papers and impressions.

The standardized colours act as selective light absorbents on the same principle as screens for trichromatic colour work, but differ from these in their having a definite standard of colour depth and colour purity.

Work of this character requires paper of a known degree of sensitiveness, but on inquiry it was found that no reliable standard had as yet been established. On this, six makes of “white” paper were purchased in the open market and submitted to exposures under the following conditions:--

Six slips of the sensitized papers were covered by a thin metal plate, pierced by six rows of apertures, a complete row lying over each sample of paper. The rows contained seven apertures each, one being left uncovered to receive the full energy of the impinging light. The remaining six were covered respectively by a Red, Orange, Yellow, Green, Blue or Violet standardized screen of 15 units colour intensity; the whole was fitted into a suitable exposure frame. Many exposures were made, from which the four following were selected:--

No. 1. 20 min. exposure to a dull sky. " 2. 10 " " " bright sunlight. " 3. 20 " " " " " 4. 30 " " " "

The results are arranged in Tables VI and VII.

Table VII contains results of different exposures under sunlight conditions.

TABLE VI.

DULL SOUTH LIGHT.

20 Minutes’ Exposure on White Sensitized Papers.

-----+-----------------+-----------------+------------------ Pap-| | | ers | OPEN SCREEN. | RED SCREEN. | ORANGE SCREEN. | | | -----+-----------------+-----------------+------------------ | Blk. Or. Red. | Blk. Or. Yel. | Blk. Or. Red. 1 | 7·8 4·2 6·0 | 2·0 4·8 3·0 | ---- ·3 ·2 | | Red. | 2 | 6·0 5·0 5·5 | 1·9 3·1 ·4 | ---- ---- ---- | | Yel. | 3 | 7·8 3·2 7·0 | 2·7 4·7 3·0 | ---- ---- ---- | | Red. | 4 | 7·2 4·3 6·0 | 2·2 3·2 ·2 | ---- ---- ---- 5 | 5·8 7·2 4·0 | 1·9 2·3 ·8 | ---- ---- ---- 6 | 5·6 7·4 4·0 | 1·8 2·6 6 | ---- ---- ---- -----+-----------------+-----------------+------------------

-----+-----------------+-----------------+-----------------+------------------ Pap-| | | | ers | YELLOW SCREEN. | GREEN SCREEN. | BLUE SCREEN. | VIOLET SCREEN. | | | | -----+-----------------+-----------------+-----------------+------------------ | Blk. Or. Red. | Blk. Or. Red. | Blk. Or. Red. | Blk. Or. Red. 1 | 2·0 3·0 1·6 | ---- ---- ---- | 3·9 7·6 3·5 | ·2 ·4 1·0 | | | Yel. | 2 | ·4 ·4 1·0 | ---- ---- ---- | 4·3 9·2 4·5 | ·2 ·8 ·8 | | | Red. | 3 | 1·2 ·5 ·8 | ---- ---- ---- | 3·7 9·3 2·0 | ·7 ·6 ·9 | | | Yel. | 4 | 1·2 ·1 1·2 | ---- ---- ---- | 3·4 1·1 1·0 | ·7 ·7 ·9 5 | 1·0 ·3 ·3 | ---- ---- ---- | 3·3 8·7 2·0 | ·3 ·3 1·2 | | | Red. | 6 | ·2 ·3 ·4 | ---- ---- ---- | 3·6 8·9 1·5 | ·4 ·2 ·9 -----+-----------------+-----------------+-----------------+------------------

TABLE VII.