Reflected solar radiation from horizontal, vertical and inclined surfaces: ultraviolet and visible spectral and broadband behaviour due to solar zenith angle, orientation and surface type.

Abstract

Ultraviolet (UV) radiation affects human life and UV exposure is a significant everyday factor that individuals must be aware of to ensure minimal damaging biological effects to themselves. UV exposure is affected by many complex factors. Albedo is one factor, involving reflection from flat surfaces. Albedo is defined as the ratio of reflected (upwelling) irradiance to incident (downwelling) irradiance and is generally accepted only for horizontal surfaces. Incident irradiance on a non horizontal surface from a variety of incident angles may cause the reflectivity to change. Assumptions about the reflectivity of a vertical surface are frequently made for a variety of purposes but are rarely quantified. As urban structures are dominated by vertical surfaces, using albedo to estimate influence on UV exposure is limiting when incident (downwelling) irradiance is not normal to the surface. Changes to the incident angle are affected by the solar zenith angle, surface position and orientation and surface type. A new characteristic describing reflection from a surface has been used in this research. The ratio of reflected irradiance (from any surface position of vertical, horizontal or inclined) to global (or downwelling) irradiance (RRG) has been calculated for a variety of metal building surfaces in winter time in the southern hemisphere for both the UV and visible radiation spectrum, with special attention to RRG in the UV spectrum. The results show that the RRG due to a vertical surface can exceed the RRG due to a horizontal surface, at smaller solar zenith angles as well as large solar zenith angles. The RRG shows variability in reflective capacities of surface according to the above mentioned factors and present a more realistic influence on UV exposure than albedo for future investigations. Errors in measuring the RRG at large solar zenith angles are explored, which equally highlights the errors in albedo measurement at large solar zenith angles.

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Reflected solar radiation from horizontal, vertical and
inclined surfaces: Ultraviolet and visible
spectral and broadband behaviour
due to solar zenith angle, orientation and surface type

J. Turner1,* A.V. Parisi1 and D. J. Turnbull1

1Faculty of Sciences, University of Southern Queensland, TOOWOOMBA. 4350.
AUSTRALIA. Fax: 61 74 6312721. Email: turnerjo@usq.edu.au
*To whom correspondence is to be addressed


Abstract

Ultraviolet (UV) radiation affects human life and UV exposure is a significant
everyday factor that individuals must be aware of to ensure minimal damaging
biological effects to themselves. UV exposure is affected by many complex factors.
Albedo is one factor, involving reflection from flat surfaces. Albedo is defined as the
ratio of reflected (upwelling) irradiance to incident (downwelling) irradiance and is
generally accepted only for horizontal surfaces. Incident irradiance on a non
horizontal surface from a variety of incident angles may cause the reflectivity can
change. Assumptions about the reflectivity of a vertical surface are frequently made
for a variety of purposes but rarely quantified. As urban structures are dominated by
vertical surfaces, using albedo to estimate influence on UV exposure is limiting when
incident (downwelling) irradiance is not normal to the surface. Changes to the
incident angle are affected by the solar zenith angle, surface position and orientation
and surface type. A new characteristic describing reflection from a surface has been
used. The ratio of reflected irradiance (from any surface position of vertical,
horizontal or inclined) to global (or downwelling) irradiance (RRG) has been
calculated for a variety of metal building surfaces in winter time in the southern

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hemisphere for both UV and visible radiation spectrum, with special attention to RRG
in the UV spectrum. The results show that the RRG due to a vertical surface can
exceed the RRG due to a horizontal surface, at smaller solar zenith angles as well as
large solar zenith angles.
The RRG shows variability in reflective capacities of surface according to the above
mentioned factors and present a more realistic influence on UV exposure than albedo
for future investigations. Errors in measuring RRG at large solar zenith angles is
explored, which equally highlights the errors in albedo measurement at large solar
zenith angles.

Keywords: albedo, RRG, vertical surfaces, UV radiation, visible, solar zenith angle

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Introduction
Exposure to biologically effective ultraviolet (UV) radiation can be beneficial to
human health in the form of initiating pre-vitamin D3 formation [1] and detrimental;
such as erythema, skin cancer, ocular damage and more [2]. UV exposure is specific
to the formation of the above health effects and much research has been conducted to
measure and model UV exposure. UV radiation is influenced by (and UV exposure
modelling must take into account) many atmospheric factors; including solar zenith
angle, altitude, latitude, ozone, clouds, aerosols, albedo (reflectivity) [3,4] and
personal factors; including occupation and personal behaviour [5-8].
Albedo is defined as the ratio of reflected (upwelling) irradiance to incident (down
welling) irradiance for horizontal surfaces [9]. A surface that varies from a horizontal
position but is still exposed to downwelling irradiance (to be referred to as global
irradiance in this paper) can still produce reflected irradiance from the surface to the
immediate environment. Due to the nature of the definition of albedo, this type of
reflectivity cannot be assumed to be equivalent to albedo as it is dependent on the
surface orientation and direction, solar zenith angle and type of surface, all of which
contribute changes in the angle of incident UV radiation. If albedo is used to
approximate this type of reflectivity for non horizontal surfaces, then contributions of
such reflectivity to UV exposures to an individual could either be underestimated or
overestimated. In the complex nature of the human environment, particularly urban
environments which are dominated by vertical surfaces, understanding the interaction
of reflected irradiance from vertical surfaces (as well as horizontal and inclined) will
be important for health and safety issues for outdoor workers.
Previous work on reflectivity includes the investigation of the albedo of horizontal
surfaces [9-12] as well as inclined surfaces such as snow covered mountain sides [13-

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14]. Irradiances on inclined surfaces affected by surrounding albedo have been
previously explored [15-17]. Investigation of the biological effectiveness of the
contribution to personal UV exposures due to the albedo of different horizontal and
inclined surfaces has been investigated [18-20]. These effects are particularly
important for outdoor workers’ who should be aware of the contribution to UV
exposures from such surfaces due to everyday working conditions [21]. The Guidance
Note for the Protection of Workers from the Ultraviolet Radiation in Sunlight [21],
specifies that workers be aware of the reflection of shiny metallic surfaces in the
worker’s vicinity. However, little information is available about the reflective
capabilities of global UV irradiances from vertical surfaces in the vicinity of outdoor
workers. The albedo of vertical surfaces has been briefly investigated for glass
surfaces, along with the effect on diffuse UV due to the presence of walls [20]. Other
studies mention albedo due to vertical surfaces in terms of modelling UV exposure
but do not define the type of reflection of the global radiation from that surface [22].
Global irradiance will vary according to the factors that influence the incidence angle
of UV irradiance on a surface. Investigation of the UV irradiance received on
vertical surfaces has been carried out [11,19,23]. Webb et al. [24] determined a
relationship between vertical and horizontal irradiances. This research found that
when solar zenith angles are large and the vertical surface is facing the direction of
the sun, the vertical surface will receive more irradiance than a horizontal surface.
The study for horizontal albedo of roofing material by Lester and Parisi [18] shows
high albedo recorded for shiny and coated horizontal surfaces, but does not consider
vertical surfaces. However, in many industrial work sites, it can be commonplace to
use the metal roof cladding as wall cladding. In urban residential areas, coated metal
surfaces are used for fencing and both coated and shiny surfaces can be used for

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garden sheds or garages. This paper will compare the ratio of reflected irradiance to
global irradiance (RRG) due to vertical and inclined surfaces to the albedo of a
horizontal surface, by considering both the spectral and broadband RRG for visible
and UV radiation. The paper will consider the variations of RRG due to solar zenith
angle, orientation of the vertical plane and different metal surface types. Since albedo
is quantitatively the same as the RRG on a horizontal surface, the term RRG will be
used instead of albedo, unless referring specifically to referenced albedo
measurements.

Methodology
The measurements were carried out on an archery field at the University of Southern
Queensland, Toowoomba, Australia (27.5° S, 151.9° E). A metal frame was
constructed to support a 1 m × 1 m size vertical sheet, of either Trimclad (trapezoidal)
or corrugated sheet metal, to simulate the exterior wall of a building or a fence. A
secondary piece of sheet metal of the same size was attached to the other side of this
metal frame, inclined at 35º from the horizontal, simulating the sheet metal on the
roof of a building. Both sheets were separated by the frame with sufficient distance
between each sheet to prevent shading.
Eight types of Trimclad metal sheeting and two types of corrugated metal sheeting
were used (supplied by Metroll, Toowoomba). The corrugated metal sheets consisted
of one Zincalume (steel coated with zinc and aluminium) surface and one Colorbond
cream coloured surface. The distance between the ridges on the corrugated iron waves
was 7.8 cm and the height difference between a trough and a peak was 1.7 cm. The
Trimclad metal sheeting consisted of one Zincalume surface, six Colorbond coated
surfaces (cream, beige, pale green, blue, dark red and black) and one Zincalume

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