VCT, method for investigation about level of light at the working table and for the complementary ambient light

The method used in the investigation of the level of light at the working table is developed with the purpose to collect data about user preferences in the combination of light at the table and in the ambient light. The method was developed by Bo Persson at KTH in Stockholm together with opticians and was further developed by the author of the Thesis and used at Jonkoping University for 10 years. In the study performed in the Thesis, there were no ambitions to use previous data or methods from earlier studies. The study is done without ambition to generalise about an optimal level, the purpose was only to demonstrate the variety of preferences in a group of subjects. After data was collected I started to read about previous studies to compare my result with others. I found that generalisations of 500 Lux as a well-functioning level of light at the working table goes back to M. Aguilar and W.S. Stiles 1954 and their work with the saturation of the Rod Mechanism of the Retina at High Levels of Stimulation [Aguilar, Stiles 1954].

“From the results obtained here by this method it is concluded that at a field intensity of about 100 scotopic trolands the sensitivity of the rod mechanism to stimulus differences begins to fall off rapidly and that at about 2000 to 5000 scotopic trolands (corresponding approximately to daylight luminances of 120 to 300 cd/m²) the rod mechanism becomes saturated and is no longer capable of responding to an increase of stimulus.” [Aguilar, Stiles 1954 p.59].

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The first study performed by Stiles was done with one subject [Stiles 1939]. Aguilar and Stiles increased the number of subjects to four in the study performed in 1954 [Aguilar & Stiles]. Miss P. Fowler, Miss S. Ide and Miss E. Kinrade acted as subjects. Based on the results of these three subjects and a fourth subject not mentioned by name, 500 Lux is found optimal by Perry, Campell and Rothwell in 1987. In a study performed by M. J. Perry, F.W. Campbell and S.E.

Rothwell in 1987, it is concluded: The current edition of CIBSE interior lighting code recommends as a standard service illumination of general office work of 500 Lux [Perry, Campbell & Rothwell 1987].

“Assuming an effective working plane reflectance range of 2.23-0.36, this equates to a log luminance range of 1.56-1.77. his range of log luminance lies reasonably close to the lower bound of the Aguilar and Stiles rod saturation region. Allowing some decrease in response time before it is deemed to affect performance, the present data show that, in terms of response times, these recommendations are optimal.

Perry, Campbell & Rothwell 1987” [Perry, Campbell & Rothwell 1987].

“This conclusion does not answer the question of why given a free choice, some people will select desk top illuminations on 1600 Lux or greater. The lack of correlation between an apparently optimum luminance and the subject’s free choice of luminance will require further investigation” [Perry,

Campbell & Rothwell 1987].

“Previous studies of an observer´s preference for different task area luminance’s and illumination levels are in reasonable agreement with the present results. Balder asked his observers to rate the acceptability of the luminance of a piece of white paper placed on a desk into one of the three categories too dark, good, too bright. Approximately 70 per cent of the observers considered that good lighting occurred for a task luminance of between 325 and 410 candelas per square meter, the maximum percentage occurring for a luminance of 365 candelas per square meter, corresponding to an illumination of 1770 Lux. In the present study such a luminance corresponds to a desk illumination level of 1.550 Lux, i.e., in the region where the preference rating is relatively constant” [Perry, Campbell & Rotwell 1987, p .42].

“Balder [Balder1957] also found that some observers found the task luminance to be too bright at levels above approximately 200 candelas per square meter (illumination level 1.000 Lux). Similar results were obtained by Bodman [Bodman, Sollner & Voit 1963] and his colleagues (10, 11) who concluded that a preferred range of task illumination levels occurred between 1.000 and 2.500 Lux. Riemenschneider (12) has obtained an optimum illumination level of the order of 2.500 Lux, although it seems possible that this relatively high level was due in part to a lack of control of the adaption conditions, similar to the effect found in the pilot experiment described in Section 2. In Balder and Bodman and the present studies the indications are that the mean observer´s preference for the lighting on a simple office type task, at least for normal task-surround conditions, is fairly constant for illumination levels between approximately 1.000 and 2.500 Lux, and that the maximum preference probably occurs in this range.” [Perry, Campbell

& Rotwell 1987 p.42].

“All three studies show that an increasing number of observers begin to find the lighting on the task too bright as the level is raised above 1.000 Lux, although such a comment is not associated with as low a preference rating as when the lighting is too dark” [Perry, Campbell & Rothwell 1987, p.42].

“In Balder´s, Bodman´s and the present studies the indication are that the mean observer´s preference is fairly constant “[Perry, Campbell & Rothwell 1987, p.42].

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Comparison with the studies in the Thesis: Study 4 and 5 in the Thesis are hard to compare with Perry´s et al. and Balder and Bodman. They do not have the same subjects, contrast situation or light settings and work tasks. The ambition in the study performed in the Thesis was to let the subjects judge their visually comfortable level of light on the table when reading black letters on a white paper and to combine this with their preference for the preferred level of complementary ambient light. The preferred level of light for the working table was measured first and is used here for the comparison.

J.E. Saunders studies [1969] can be related to the result of Balder and Bodman.

“In Balder´s, Bodmans and the present studies the indications are that the mean observer’s preference for the lighting on a simple office-type task at least for normal task-surround conditions, is fairly constant for illumination levels between 1.000-2.500 Lux and that the maximum preference probably occurs in this range” [Saunders 1969].

When comparing the result of the five studies, I find that, despite their being different, they point in the same direction for the most preferred level of light. Saunders,1969 (1550 Lux ) Balder, 1957 (1770 Lux), Bodman, 1967 (1000-2500 Lux), Riemenscheder, 1967 (2500 Lux), Säter 2011 (M=2007 Lux). However, in order to be able to compare these five studies in an accurate way, they need to be reconstructed and used in the same way by the same subjects, as well as

measured in the same way and this is not done here.

3.1.7 Summary

Self-evaluation with semantic scales and physiological markers are used in the studies. Those are the most frequent methods seen used today. They are restricted in many ways. The self- evaluation depends on if the subjects are naïve or trained and need to be related to the individual’s disposition for receiving EMR, visual performance and diurnal rhythm. Methods can be compared in the way they are performed but also in the theory that lies behind the work with the development of the methods. I find the result of Wever, Hollwich, Ronchi and Roenneberg et al. important in many ways. They are usable for predictions of the public´s relation to light. It is difficult to imagine the effort needed for Hollwichs´ 30 000 blood samples, Wevers´ night and day shifts 1964-1989 and Roenneberg and his colleagues work with 55,000 subjects. Luckiesh and Moss 100,000 measurements and Ronchis´ work for 30 years. But this is the effort needed to be able to predict human responses to PS. The studies in the Thesis are done with the ambition to look closely on a small group of subjects and to see the actual individual response and preference. The studies performed in the Thesis are small prestudies far from the mappings for normal distribution curves done in the studies of Roenneberg et al., Wever and Hollwich. Lucia Ronchis´ studies since 1960 show the complexity in the visual function as a part of the human body in a diurnal rhythm going from deepest sleep to the highest level of alertness.

The studies performed in the thesis and by Luckiesh and Moss, Wever, Hollwich, Roenneberg et al. and Lucia Ronchi share the knowledge about the importance of the light-related diurnal rhythm and show the differences between the individual users. This all fits in to the holistic theory of health of Pörn. The methods used are different in the way they are performed and are

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by that not possible to compare to each other. Despite this, the methods used have in common that they investigate the interaction of MLCS and the huge inter-individual differences among subjects when staying in the same interaction model.