BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and equipment for controlling the luminance
of an illuminated surface, the method comprising the step of controlling the luminance
of an illuminated surface.
[0002] When illuminating surfaces, such as signs, the luminance of the surface must be above
a certain level. This way, the illuminated surface stands out from its environment
in all conditions. Earlier, the illumination of signs and the like was arranged by
allowing the illuminating light sources to burn at the brightness achieved by using
a constant voltage, for instance, which feeds the light sources. In the beginning,
the light sources then produce an unnecessarily high illuminance, since in most cases,
even a lower illuminance would suffice to produce the necessary luminance. A LED diode,
for instance, is a typically used light source. The illuminance of LED diodes decreases
with time, and when used as described, the luminance achieved by the light source
decreases and finally falls below a predefined level. Exact information on the falling
of the luminance below a desired level cannot be obtained, which means that replacing
the light sources must be done by estimate or after they have gone out completely.
[0003] Prior art does not take into consideration the luminance of the illuminated surface
produced by external light or the brightness of the environment of the illuminated
surface produced by external light. External light, such as room lighting or sunlight,
may illuminate the environment of the illuminated surface so brightly that the illuminated
surface does not stand out clearly enough from its environment. It would then be desirable
to increase the illumination of the illuminated surface to a certain limit while the
illumination of the environment increases. This way, the service life of the illuminating
elements, such as LED diodes, would be increased, because the illuminating element
needs not burn at full power all the time.
BRIEF DESCRIPTION OF THE INVENTION
[0004] It is an object of the present invention to provide a method which avoids the above-mentioned
drawbacks and enables controlling the luminance of the illuminated surface in an appropriate
and reliable manner. This object is achieved by a method of the invention characterized
in that the method also comprises the steps of
defining the brightness of the environment of the illuminated surface to achieve
a reference brightness and controlling the luminance of the illuminated surface with
an illuminating element depending on the level of the reference brightness.
[0005] The method of the invention is based on the idea that by defining the brightness
of the environment of the illuminated surface and by controlling the luminance of
the illuminated surface according to this brightness, a desired contrast is achieved
between the illuminated surface and its environment. The luminance is then controlled
according to the changing brightness properties of the environment so that when the
illumination of the environment increases, the illumination of the surface illuminated
by an illuminating element is also increased, and correspondingly, when the illumination
of the environment decreases, the illumination of the illuminated surface is also
decreased. The method provides a considerable advantage in that the contrast between
a continuously illuminated surface, such as an exit sign, and its environment is kept
within a certain limit. With respect to prior art, this method provides a considerable
increase in the service life of the illuminating element, since the element is used
at a power which is adequate to produce the required contrast between the illuminated
surface and its environment.
[0006] The invention also relates to equipment for controlling the luminance of the illuminated
surface, which equipment comprises an illuminating element which is arranged to illuminate
the illuminated surface. The equipment is characterized in that it also comprises
an illumination sensor which is arranged to define the brightness of the environment
of the illuminated surface, and control equipment which is arranged to control the
illuminance produced by the illuminating element in response to the brightness of
the environment of the illuminated surface. With the equipment, it is possible to
achieve the advantages provided by the method of the invention using a simple and
reliable structure. The equipment of the invention for controlling luminance is especially
suited for use in exit signs in premises where the lighting is occasionally dimmed,
such as in cinemas and theatres.
BRIEF DESCRIPTION OF THE FIGURES
[0007] In the following, the invention will be described in greater detail by means of preferred
embodiments and with reference to the attached drawings in which
Figure 1 shows a schematic of the equipment implementing the method of the invention,
and
Figure 2 is a graphical presentation of the luminance of an illuminator implementing
the method of the invention when the brightness of the environment increases.
DETAILED DESCRIPTION OF THE INVENTION
[0008] Figure 1 shows a schematic of an illuminated surface 1 and the components arranged
with it to implement the method of the invention. The illuminated surface can be a
sign, such as an exit sign or the like, which should stand out from the illumination
conditions of the environment. For illuminating the illuminated surface, the equipment
comprises an illuminating element 5 which is made up of LED diodes, for instance.
The purpose of the illuminating element is to illuminate the surface to be illuminated
in such a manner that the surface achieves a predefined luminance in relation to the
brightness of the environment of the illuminated surface.
[0009] The equipment of the invention shown in Figure 1 also comprises an illumination sensor
7 which enables the definition of the illumination of the environment of the illuminated
surface. The sensor 7 only determines the illumination of the environment and it is
arranged so that the light reflecting from the illuminated surface does not reach
the sensor detecting external light. To achieve this, the sensor can be encased or
located in a suitable manner. According to the invention, the sensor 7 defines the
brightness of the environment of the illuminated surface to produce a reference brightness.
The reference brightness refers to a reference signal proportional to the brightness
of the environment. The amount of light produced by the illuminating element is adjusted
in relation to the reference variable defined in this manner. In other words, as the
reference brightness increases, the luminance of the illuminated surface is increased
to maintain a certain contrast between the surface and its environment. In doing this,
it is obvious that the control equipment of the illuminating element must contain
listed value for reference brightness, for instance, and values of the illuminating
element current corresponding to them or the like, by means of which a desired luminance
can be made to correspond to a certain reference brightness.
[0010] According to a preferred embodiment of the invention, the luminance of the illuminated
surface is also defined. An illumination sensor 2 is used to define the luminance.
The illumination sensor 2 defines in its entirety the brightness which is formed on
the illuminated surface by the illuminating element and possible external light. According
to the embodiment, the luminance of the illuminated surface is defined to achieve
a luminance signal, and the luminance of the illuminated surface is controlled with
the illuminating element using control equipment 3 depending on the level of the reference
brightness and the strength of the luminance signal. According to the embodiment,
there are two separate signals available, whereby controlling the luminance is simple
based on comparing the signals. The luminance produced by the illuminating element
5 is controlled in direct proportion to the reference brightness, for instance. In
other words, as the illumination of the environment of the illuminated surface changes,
the amount of light produced by the illuminating element and hitting the illuminated
surface is changed accordingly. A situation is achieved by this, in which the contrast
between the surface and its environment remains constant. A constant contrast ensures
that the illuminated surface, such as an exit sign, stands out clearly.
[0011] It is, however, necessary to limit the illumination produced by the illuminating
element, if the brightness of the environment of the surface is exceptionally high.
This limitation can be done for instance by measuring the magnitude of the current
to the illuminating element and limiting the current to a predefined maximum value.
The purpose of the limitation is to ensure that the illuminating element is not worn
unnecessarily quickly as a result of excess current. Limiting the amount of light
produced by the illuminating element is shown graphically in Figure 2, in which a
thicker continuous line 10 shows the brightness of the environment of the illuminated
surface and a thin continuous line 11 shows the luminance achieved at the brightness
in question. In the case shown by the figure, the brightness 10 of the environment
of the illuminated surface increases linearly with time, and the figure shows the
control of the luminance of the illuminated surface according to a preferred embodiment
of the method. In the example of the figure, when the brightness of the environment
is first at its minimum, i.e. dark, the illuminated surface must, however, be illuminated
to ensure visibility. As the brightness of the environment increases above a predefined
lower limit 13, the luminance of the surface is also increased from a first constant
value 15 by illuminating it with the illuminating element. When the brightness of
the environment is between the lower limit 13 and an upper limit 12, the luminance
of the surface is adjusted so that a desired contrast is maintained between the surface
and the environment. In the example, the luminance of the surface is kept lower than
the brightness of the environment, but the luminance of the surface can also be higher
than the brightness of the environment. In addition, in the example, the luminance
of the illuminated surface is increased linearly, but this can also be done stepwise.
[0012] As the brightness of the environment further increases, the upper limit 12 is reached,
and increasing the luminance is stopped using the illuminating element. In spite of
the possible increase in the brightness of the environment, the luminance of the illuminated
surface is then maintained at a second constant value 14. The upper limit for the
luminance produced by the illuminating element can, for instance, be determined in
relation to the magnitude of the current of the LED diodes used for illumination in
such a manner that an excess current which would unduly load the LED diodes is not
allowed through them.
[0013] If the illuminated surface is an exit sign, it is important that the surface has
a certain minimum luminance at all times to maintain its visibility. Figure 2 shows
that when the brightness of the environment is zero or close to zero, the luminance
is maintained at a constant level which is the minimum luminance.
[0014] As the illuminating element, such as a LED diode, ages, its luminosity decreases.
It is natural that the maximum luminosity produced by a diode at a maximum magnitude
of current also decreases with time. Therefore, according to an embodiment of the
invention, an alarm signal is produced, if the desired luminance is not achieved.
The alarm signal provides advance warning of the fact that the condition of the illuminating
element has deteriorated considerably and the element needs to be replaced. The alarm
signal can be transmitted to a person responsible for maintenance by visual means,
for instance, using an alarm element 4, such as a LED diode or another signal light
which can be located with the control equipment 3 controlling the illumination of
the illuminated surface.
[0015] An alarm signal can also be produced if the brightness of the environment of the
illuminated surface is so high that it is not possible to produce the desired contrast
between the surface and its environment using the illuminating element.
[0016] Control equipment located with the illuminated surface can in practice control the
illuminance by adjusting the current running through the illuminating element. Not
only is the current adjustment simple to implement, but at the same time also the
magnitude of the current running through the illuminating element will be measured,
and the current can be limited to a predefined value.
[0017] It is obvious to a person skilled in the art that while technology advances, the
basic idea of the invention can be implemented in many different ways. The invention
and its embodiments are thus not restricted to the examples described above, but can
vary within the scope of the claims.
1. A method of controlling the luminance of an illuminated surface, the method comprising
the step of controlling the luminance of an illuminated surface (1),
characterized in that the method also comprises the steps of
defining the brightness of the environment of the illuminated surface (1) to achieve
a reference brightness, and
controlling the luminance of the illuminated surface with an illuminating element
(5) depending on the size of the reference brightness.
2. A method as claimed in claim 1,
characterized in that the method also comprises the steps of
defining the luminance of the illuminated surface (1) to produce a luminance signal,
and
controlling the luminance of the illuminated surface with an illuminating element
depending on the level of the reference brightness and the strength of the luminance
signal.
3. A method as claimed in claim 1 or 2, characterized in that the luminance of the illuminated surface (1 ) is adjusted in direct proportion
to the reference brightness.
4. A method as claimed in claim 1, 2 or 3, characterized in that the method also comprises the step of producing an alarm signal if the desired
luminance of the illuminated surface is not reached.
5. A method as claimed in any one of claims 1 to 4, characterized in that the method also comprises the step of limiting the luminance produced by
the illuminating element (5) when the desired luminance exceeds a predefined luminance
limit.
6. A method as claimed in any one of claims 1 to 5, characterized in that controlling the luminance of the illuminated surface comprises the step of
adjusting the current running through the illuminating element (5).
7. A method as claimed in claim 6,
characterized in that producing the alarm signal comprises the steps of
defining the magnitude of the current running through the illuminating element, and
producing an alarm signal when the defined magnitude of current exceeds a predefined
limit.
8. A method as claimed in claim 1 or 2,
characterized in that controlling the luminance of the illuminated surface comprises the steps
of maintaining the luminance of the illuminated surface at a first constant value
(15) when the brightness of the environment is below a predefined lower limit value
(13),
maintaining the luminance of the illuminated surface at a second constant value (14)
when the brightness of the environment is above a predefined upper limit value (12),
and
controlling the luminance of the illuminated surface linearly or stepwise when the
brightness of the environment is between said lower and upper limit value (12, 13)
so that a predefined contrast remains between the illuminated surface and its environment.
9. Equipment for controlling the luminance of an illuminated surface, the equipment comprising
an illuminating element (5) arranged to illuminate the illuminated surface (1), characterized in that the equipment also comprises an illumination sensor (7) arranged to define
the brightness of the environment of the illuminated surface, and control equipment
(3) arranged to control the illuminance produced by the illuminating element in response
to the brightness of the environment of the illuminated surface.
10. Equipment as claimed in claim 9, characterized in that the equipment also comprises an illumination sensor (2) arranged to define
the luminance of the illuminated surface (1), and that the control equipment is arranged
to control the illuminance produced by the illuminating element in response to the
brightness of the environment of the illuminated surface and the luminance of the
illuminated surface.
11. Equipment as claimed in claim 9 or 10, characterized in that the equipment also comprises an alarm element (4) arranged to produce an
alarm signal if the desired luminance is not reached.
12. Equipment as claimed in any one of claims 9 to 11, characterized in that the alarm element (4) is arranged to produce an alarm signal if the current
running through the illuminating element exceeds a predefined limit.
13. Equipment as claimed in any one of claims 9 to 12, characterized in that the illuminating element (5) comprises one or more LED diodes.
14. Equipment as claimed in any one of claims 9 to 13, characterized in that the alarm element (4) comprises one or more LED diodes.