• diffuse irradiance angular distribution
• transposition models
• solar exposure
• erythema
• UV
• ultraviolet

The ultraviolet band (UV) of solar spectrum has a crucial impact on human health and
assessing personal UV exposure is essential for evaluation of the related risks and benefits for
public health outcomes and for epidemiological studies.
UV radiation is the main environmental non-genetic cause of skin cancer but, at the same
time, it triggers the mechanism of vitamin D3 production in the skin, which is an hormon associated with bones and immune system health. Overexposing, and resulting sunburns, translates
into an increased risk of a skin cancer development in later life. It’s also true that a low chronical UV exposure is linked to vitamin D3 deficiency or insufficiency, which is associated with
rickets, osteomalacia and osteoporosis. As a matter of facts a balanced exposure is appropriate
for getting an overall positive effect and the present work goes along with this approach. Indeed
it’s focused on skin cancers prevention through a controlled sun exposure attitude that will let
people benefit and avoid risks coming from natural UV radiation.
In the field of skin cancers prevention, it’s common practise considering the erythemal biological effectiveness of solar radiation spectrum. The Commission International de l’Eclairage (CIE),
collaborating with the International Organization for Standardization (ISO), provided a worlwide recognized erythema action spectrum; this action spectrum will be either mathematically
or physically applied to measurements taken throughout this work.
Obtaining real time personal UV exposure is a great challenge because of two main reasons:
1) the intensity and spectral composition of UV radiation is time and space dependent and 2) the
human body on which UV radiation is incident is made up by surfaces differently oriented towards
the Sun. The company SiHealth exploits satellite data in order to fulfill the first point while the
present work will uniquely deal with the second issue, whose difficulty relies on modelling diffuse
UV radiation angular distribution over the sky dome.
1
The diffuse radiation is defined as the part of radiation that reaches Earth ground as a
result of being scattered by particles in the atmosphere. Therefore, restricting to UV range and
taking into account Rayleigh scattering, whose cross section has a dependance of ∼ λ−4, the
diffuse component is a relevant fraction of UV total radiation. For example in Harwell campus,
UK (which is the place where experiments were undertaken) the ratio of diffuse to total UV
radiation observed during this work over various metereological conditions was at least 45%.
In the past years few models describing diffuse angular distribution over the sky dome were
developed for energy applications and so they were meant to be applied to the entire solar
spectrum. In the present work a selection of these models will be applied only to UV range
in order to retrieve how they perform in this spectral band over mixed sky conditions. The
principal assumption made is that, knowing at any given time and location total and diffuse
horizontal irradiance, it’s possibile to obtain irradiance incident on tilted surfaces differently
oriented (which simulate the different orientations of human body surfaces). As a results of that,
all these models have as inputs, other than Sun locating angles, measured global and diffuse
horizontal irradiances.
Ground-based measurements of erythemal weighted irradiance were simultaneously taken
on 8 different spatial orientations in order to provide experimental data of irradiance angular
distribution. These values will be compared with those obtained by the models on the same
orientations and at the same time. Building this experimental dataset is the result of 6 months
of collaboration with the Laser and Optical Radiation Dosimetry Group of the Public Health
England (PHE) Centre for Radiation, Chemical and Environmental Hazards located in Harwell
Campus, Oxfordshire, UK. PHE provided instruments, facilities and experience to take experimental measurements over Spring and Summer 2018. In summary the present work includes:
calibrating different instruments and monitoring their performances, taking measurements of erythemal weighted irradiance on different orientations, getting models’ inputs, retrieving related
outputs and performing data analysis comparing experimental and modelled values.