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Year : 1991  |  Volume : 57  |  Issue : 1  |  Page : 17-18

Exposure time to sunlight for ultra violet light therapy

CR Srinivas, Rama Devadiga, TR Parashar, Chandras 

Correspondence Address:
C R Srinivas


Exposure time to sunlight for photo or photochemotherapy is arbitrarily determined. This could lead to inadequate or over exposure. As a sequence to our initial pilot study on exposure time for PUVASOL, we have now developed a bar chart which could be used as a guide to achieve a more logical and uniform exposure time during different months. We plan to extend the study to different zones in India.

How to cite this article:
Srinivas C R, Devadiga R, Parashar T R, Chandras. Exposure time to sunlight for ultra violet light therapy.Indian J Dermatol Venereol Leprol 1991;57:17-18

How to cite this URL:
Srinivas C R, Devadiga R, Parashar T R, Chandras. Exposure time to sunlight for ultra violet light therapy. Indian J Dermatol Venereol Leprol [serial online] 1991 [cited 2020 Oct 21 ];57:17-18
Available from: https://www.ijdvl.com/text.asp?1991/57/1/17/3610

Full Text

Sunlight at the crust of our planet is con­tinually modified by the (1) time of the day (2) the season of the year (3) the observers lati­tude on the Earth's surface, (4) his altitude above the sea level (5) the weather and atmo­spheric pollution. [1] The above factors should be taken into consideration while determining the duration for exposure to sunlight for Ultraviolet Phototherapy. Arbitrary exposure times may lead to inadequate or over exposures. [2] This study is an attempt at semi quantifying exposure times to sunlight for photo and photochemotherapy.

 Materials and Methods

UVA (320-400 nm) and UVB (290-320 nm) were measured at weekly intervals during the year 1989. UVA was measured with a UVA monitor LMA - 302 National Biological Corpo­ration while UVB was measured with UVB LMB 066. Meter reading was recorded at 11 AM and 3 PM and the readings were recorded for 60° E and 120° W in relation to the position of the sun. [2] Readings were not taken during the months of July, August and September because of heavy clouds and unpredictable weather. All the data was stored in the computer which was programmed to calculate the exposure time to sunlight for one joule of UVA or 10 millijoule of UVB. using the formula.

[INLINE:1] The graphics of the computer was used to print out a comprehensive table showing the expo­sure time during each season.


The exposure time to sunlight for UVA and UVB at 11 AM and 3 PM respectively is shown in [Table 1] and [Table 2].


In our pilot study we had published expo­sure time to sunlight for PUVASOL for a one month. [2]sub This study is a more comprehensive study carried out between June 1988 to May 1989 and included meter reading of both UVA and UVB. It is important to take both into con­sideration as both photo augmentation [4] and photo addition [5],[6] has been reported. The known erythemogenic property of high dose of UVA may be additive to subclinical or visible erythema induced by UVB [6]. The normal dose requirement of UVA is usually 6 to 12 J/cm 2 with increment of 5 to 1 J upto a maximum of 18 J/cm 2. The other more logical approach would be to determine the minimal phototoxic dose (MPD) and then gradually increase the UVA dosage. However we have consistently failed to demonstrate MPD in Indian Skin (Personal observation). It is easier to demonstrate minimal erythema dose (MED) to UVB. [7] Based on our studies we have recommended 40 mj for light colored individuals and about 70 or 80 mj for dark skinned patients. [6] Once the initial dose is ascertained, using the bar diagram [Table 1][Table 2] it is possible to decide on the exposure time and increments during each siting. For example a patient will require 110 sec exposure for 10 mj/cm 2 of UVB during the month of January whereas he will to have expose only for about 80 sec during June for the same dose of UVB [Table 2].

We recommend longer exposure during June as monsoon visits our coast during this month. The readings are likely to be different in other parts of the country as maximum intensity of UV light can be expected during June.

We hope to extend this study to various places in India and thus aim at a more logical phototherapy using sun as the source of UV light.


1Magnus IA: Sunlight in, Dermatologic Photobiology-clinical and experimental aspects, Blackwell Scientific Publications, Oxford, 1976, p 35-40.
2Srinivas CR, Devadiga R Rajeev VK et al: Exposure time to sunlight for PUVASOL, Ind J Dermatol Venereol Leprol, 1989; 55; 373-374.
3Willis I: Phototherapy: Light sources, in : Physical modalities in dermatologic therapy, Editr, Goldschmidt H: Springr-Verlag, New York, 1978; p 228-235.
4Willis I, Kligman A and Epstin S: Effects of long wave ultraviolet rays on human skin: photoprotective or photoaugmentive J Invest Dermatol, 1973; 59 : 416-420.
5Adams E, Barnes B and Forsythe W: Erythema due to ultraviolet radiation, J opt Soc Am, 1931; 21: 207-222.
6Parrish JA, Ying CY, Pathak MA and Fitzpatrick TB: Erythemogenic properties of long wave ultraviolet light in: Sunlight and man, Editors Fitzpatrick TB, Pathak MA, Harber LC et al: University of Tokyo Press, Tokyo 1974; p 131-141.
7Sandhya A, Srinivas CR Shenoi SD Balchandran C, Devadiga R: Determination of Minimal Eryghema Dose in Indian patients for Ultraviolet B phototherapy, Ind J Dermatol Venereol Leprol (In Press).


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