Radiation-dose indicator and lamp and tanning apparatus comprising such a radiation-dose indicator

Radiation-dose indicator as well as a lamp and a tanning apparatus comprising such a radiation-dose indicator. The invention provides a novel type of radiation-dose indicator which is reliable and accurate. The indicator comprises an optically active layer which includes a liquid-crystalline material as well as a radiation-sensitive compound. This compound is converted under the influence of radiation in such a way as to change the order of the liquid-crystalline material. Preferably, a trans-isomer is used as the radiation-sensitive compound, which is converted to the corresponding cis-isomer under the influence of radiation. In this way, the order of the liquid-crystalline material is broken. Indicators in accordance with the invention can very advantageously be used in UV- and IR-lamps as well as in tanning apparatus.

FIELD OF THE INVENTION 
The invention relates to a radiation-dose indicator comprising an optically 
active layer. The invention also relates to a lamp comprising such a 
radiation-dose indicator. The invention further relates to a tanning 
apparatus which is provided with such a radiation-dose indicator. 
BACKGROUND OF THE INVENTION 
Radiation-dose indicators are used in fields of application where it is 
important to total the amount of radiation of a specific wavelength range 
as a function of time. This wavelength range may be in the infrared (IR), 
visible or ultraviolet (UV) portion of the electromagnetic spectrum. Such 
a measurement may be important to determine the overall useful life of 
lamps, such as IR- or UV-lamps. By virtue thereof, the time when the lamp 
should be replaced can be indicated in a simple manner. A radiation-dose 
meter can also be used in fields of application where people are 
irradiated to determine the amount of radiation received per individual 
treatment or per series of treatments. 
A radiation-dose indicator of the type mentioned in the opening paragraph 
is known per se. For example, in the abstract of the published Japanese 
Patent Application JP 63-160.146 a description is given of a 
radiation-dose indicator which is provided on a gas-discharge lamp. The 
operation of the indicator is based on discoloration of colored indicator 
paper under the influence of radiation. The expected life of the lamp has 
almost ended when the indicator paper has discolored to such a degree that 
its color is the same as that of a reference paper. 
The known indicator has drawbacks. First of all, the reliability and 
accuracy of the known indicator are relatively low. This disadvantage is 
shared by most radiation indicators which are based on a bleaching and/or 
discoloring action. In addition, a separate reference indicator is 
necessary. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a radiation-dose indicator 
which does not have the above disadvantages. The invention more 
particularly aims at providing a radiation-dose indicator which combines a 
high reliability with a high accuracy. The indicator in accordance with 
the invention should also be cheap. The invention further aims at 
providing a lamp as well as a tanning apparatus which are provided with 
such a radiation-dose indicator. 
These and other objects of the invention are achieved by a radiation-dose 
indicator of the type mentioned in the opening paragraph, which is 
characterized in that the optically active layer includes a 
liquid-crystalline material as well as a radiation-sensitive compound 
which is converted under the influence of radiation in such a way as to 
change the order of the liquid-crystalline material. 
The invention is based on the insight that by combining radiation-sensitive 
compounds and liquid-crystalline materials, radiation-dose indicators can 
be manufactured in a relatively simple and cost-effective manner. 
Indicators of this type also have a high reliability and a good accuracy. 
The operation of the indicators in accordance with the invention is based 
on the transition between the ordered state and the unordered state of the 
liquid-crystalline material of the optically active layer. In the ordered 
or anisotropic state the liquid-crystalline material is opaque, whereas in 
the unordered or isotropic state the material is optically transparent. 
This transition can be induced under the influence of radiation-sensitive 
compounds. These compounds are converted under the influence of radiation. 
The conversion products of these compounds have such a structure that they 
change the molecular order of the liquid crystalline material. This change 
can be, on the one hand, a conversion from the ordered to the unordered 
state and, on the other hand, a conversion from the unordered to the 
ordered state. In the former situation the opaque, anisotropic, 
liquid-crystalline material is converted to transparent, isotropic 
material after it has received a certain amount of radiation. In the 
latter situation the transparent, isotropic, liquid-crystalline material 
is converted to opaque, anisotropic material after it has received a 
certain amount of radiation. 
The time when the indicator changes depends, inter alia, on the sensitivity 
of the compound to the measured radiation, the concentration of the 
radiation-sensitive compound in the optically active layer, the type of 
liquid-crystalline material of the optically active layer, the thickness 
of the optically active layer, the operating temperature of the inventive 
indicator and the intensity of the measured radiation. In particular said 
parameters enable those skilled in the art to construct, in a simple 
manner, an indicator which is suitable for the intended application. 
As stated hereinabove, in principle two types of transitions of the 
liquid-crystalline material are possible, namely from the unordered state 
to the ordered state and conversely. In practice it has been found that 
the transition from the ordered state to the unordered state is easiest to 
realize. A preferred embodiment of the indicator in accordance with the 
invention is therefore characterized in that the optically active layer 
exhibits a liquid-crystalline order which is broken after conversion of 
the radiation-sensitive compound. 
Good results have been achieved with indicators in which the 
radiation-sensitive compound decomposes under the influence of radiation. 
Examples of such compounds are tolane compounds whose conformation does 
not disturb the order of the liquid-crystalline material. A few suitable 
examples from this class are the formulas 1 (Cr 28S37N44.9I), 2 (Cr 
22S28I), 3 (Cr 58I) and 4 (Cr 40N57I), as indicated in FIG. 5. These 
compounds decompose under the influence of UV radiation. The resultant 
decomposition products disturb the liquid-crystalline order. 
Radiation-dose indicators whose optically active layer comprises a tolyl 
compound in combination with liquid-crystalline cyanobiphenyl compounds 
proved to be very suitable. Two very suitable cyanobiphenyl compounds are 
shown in FIG. 5 in the formulas 5 (Cr21S33N40I) and 6 (Cr 14N28I). The 
abbreviations placed in parenthese denote the transition temperatures of 
the various liquid-crystalline phases, such as the crystalline (Cr) phase, 
the cholesteric (S) phase, the nematic (N) phase and the isotropic (I) 
phase. 
Another preferred embodiment of the radiation-dose indicator is 
characterized in accordance with the invention in that a trans-isomer is 
used as the radiation-sensitive compound, which is converted to the 
corresponding cis-isomer under the influence of radiation. It has been 
found that such a conversion of trans-isomers to cis-isomers strongly 
disturbs the molecular order of the liquid-crystalline material. The more 
or less rod-shaped structure of the trans-isomer better fits in with the 
liquid-crystalline order than the more or less banana-shaped structure of 
the corresponding cis-isomer. In the case of these cis/transisomers, the 
point of time at which the indicator change takes place can be checked 
more accurately than in the case of the above-mentioned compounds which 
decompose under the influence of radiation. 
A class of radiation-sensitive trans-isomers which can very advantageously 
be used in the indicators in accordance with the invention has a chemical 
structure in accordance with formula 7 in FIG. 5. In this formula, A 
and/or B stand(s) for the substituents C.sub.n H.sub.2n+1 --, C.sub.n 
H.sub.2n+1 O--, C.sub.n H.sub.2n+1 --Ph --(Ph is phenyl), C.sub.n 
H.sub.2n+1 O--Ph--, C.sub.n H.sub.2n+1 --Hx--(Hx is cyclohexyl), C.sub.n 
H.sub.2n+1 O--Hx--, C.sub.n H.sub.2n+1 --Ph--C(O)O--Ph--, C.sub.n 
H.sub.2n+ --Ph--C(O))--, C.sub.n H.sub.2n+1 O--Ph--C(O)O--,C.sub.n 
H.sub.2n+1 O--Ph--C(O)O--Ph-- or C.sub.n H.sub.2n+1 
CH(CH.sub.3)--(CH.sub.2).sub.m O)--, wherein n=1-12 and m=0-6. A may be 
equal to B. A and/or B may alternatively stand for a chlorine group or a 
cyano group. X and Y stand for the groups C(H), C(CH.sub.3), C(Cl), C(CN) 
or N. X may be equal to Y. 
The aromatic rings of this class of compounds, which are coupled by the 
unsaturated X--Y bond, undergo isomerization from the trans-compound to 
the cis-compound under the influence of radiation in the incident 
absorption band of the trans-compound. This absorption band should 
therefore be in the portion of the spectrum of which the radiation dose is 
measured. 
Another preferred embodiment of the indicator is characterized in 
accordance with the invention in that the liquid-crystalline material as 
well as the radiation-sensitive compound are dispersed in an isotropic 
polymeric matrix. A material of this composition is customarily referred 
to as a PDLC-material (Polymer Dispersed Liquid Crystal). As the 
liquid-crystalline material mixes poorly with the matrix, the 
liquid-crystalline material comprising the radiation-sensitive compound is 
present in the optically active layer in the form of small droplets which 
are surrounded by the isotropic polymeric matrix. The use of a PDLC-layer 
instead of an optically active layer, which is mainly composed of 
liquid-crystalline material, has some important advantages. For example, 
the PDLC material in the indicator in accordance with the invention is 
cheaper because less liquid-crystalline material is needed. Moreover, in 
this embodiment the optically active layer can be manufactured so as to be 
a self-supporting layer. 
Isotropic polymers which can suitably be used as a matrix material have a 
refractive index which is substantially equal to that of the 
liquid-crystalline material in the isotropic state. Aromatic polymers such 
as polystyrene and polycarbonate, which can be processed from the melt or 
from solution proved to be very suitable. The most interesting matrix 
polymers, however, are thermocuring formulations such as epoxies or 
photo-curing formulations such as polyfunctional (meth)acrylates. By a 
good choice of the monomers, the refractive index of the matrix can be 
made equal to that of the isotropic liquid-crystalline material. An 
additional advantage of the use of polyfunctional monomers is that the 
optically active layer manufactured with said polymers has a good 
mechanical stability. The ratio (measured in wt. %) between the quantity 
of liquid-crystalline material and the quantity of isotropic polymer in 
the optically active layer preferably ranges between 0.5 and 2. The 
optimum ratio is approximately 1. 
A further preferred embodiment of the indicator in accordance with the 
invention is characterized in that the molecules of the liquid-crystalline 
material form part of a polymeric structure via covalent bonds. In 
combination with a radiation-sensitive compound, this so-called 
"liquid-crystalline polymer" can form the optically active layer. However, 
the polymer and the compound are preferably dispersed in an isotropic 
polymeric matrix. This combination is commonly referred to as PDLCP 
(Polymer Dispersed Liquid Crystal Polymer). As the liquid-crystalline 
groups are linked to a polymer via covalent bonds, these groups cannot 
readily escape from the optically active layer, for example by evaporation 
or exudation. This problem occurs, in particular, when the indicators are 
used at high temperatures, for example when they are provided on the 
transparent envelope of a lamp. 
If the indicator of this embodiment comprises an optically active layer of 
PDLCP-material, the material of the polymeric structure must meet a few 
requirements. It must be immiscible or poorly miscible with the material 
of the isotropic polymeric matrix. In addition, the refractive index of 
the material of the polymeric structure in the isotropic phase should be 
approximately equal to that of the isotropic polymeric matrix. It is noted 
emphatically, however, that the effect of the inventive measure of this 
embodiment is also attained if the optically active layer comprises 
exclusively, or substantially exclusively, liquid-crystalline material and 
a radiation-sensitive compound. 
A further interesting embodiment of the radiation-dose indicator in 
accordance with the invention is characterized in that the molecules of 
the radiation-sensitive compound form pan of the polymeric structure via 
covalent bonds. By virtue of this measure, it is precluded that the 
radiation-sensitive compound is exuded from the optically active layer. 
This leads to a further increase of the accuracy and reliability of the 
indicator in accordance with the invention. 
Preferably, siloxanes are used as the material for the polymeric structure. 
An important advantage of siloxanes over many other polymers is that they 
exhibit a good thermal and mechanical stability and hence ageing takes 
place relatively slowly. The liquid-crystalline groups and, if applicable, 
the radiation-sensitive compounds are preferably linked to the polymeric 
structure as side groups. This leads to an increased mobility of these 
groups and hence to an increased sensitivity of the change of the 
indicator. If a linear siloxane is used, the polymeric structure 
preferably comprises 5 to 100 monomeric units. In this range the 
properties of the polymeric structure are optimal. If, however, a cyclic 
siloxane is used, then the polymeric structure should preferably comprise 
3 to 7 monomeric units to attain optimum properties. 
A user-friendly and hence very preferred embodiment of the indicator in 
accordance with the invention is characterized in that the optically 
active layer is applied to a colored substrate and in that the optically 
active layer comprises a dye which contrasts with the color of the 
substrate. This embodiment makes it possible to use any desired color 
combination instead of the combination white/transparent. Before the 
indicator change takes place, the color of the optically active layer 
dominates, whereas after said change of the indicator the color of the 
substrate will dominate. The substrate does not have to be uniform in 
color. It is alternatively possible to use a transparent substrate which 
is provided with a colored text or colored characters. This text or these 
characters become visible after the indicator has received the intended 
amount of radiation. 
The invention also relates to a lamp. This lamp is characterized in that it 
is provided with a radiation-dose indicator as described hereinabove. Said 
lamp can be a discharge lamp, in which a discharge arc is generated 
between electrodes in a discharge tube during operation of the lamp. 
Examples of such lamps are high-pressure discharge lamps, such as 
high-pressure Na, Hg or metal-halide lamps, or low-pressure discharge 
lamps such as fluorescent lamps or low-pressure Na lamps. In this case, 
the indicator may be secured to said discharge tube. However, the lamp may 
alternatively be an incandescent lamp comprising an incandescent body in a 
vacuum envelope. The envelope can be evacuated contain a filler gas. Said 
filler gas may be inert or based on a halogen gas. Halogen lamps are a 
good example of the latter lamp type. In this case the indicator may be 
provided on the lamp envelope. 
The invention also relates to a tanning apparatus. Said apparatus is 
characterized in that it comprises a radiation-dose indicator as described 
hereinabove. Said indicator is provided on a part of the apparatus which, 
during operation of the apparatus, is not covered by the body of the user.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1-A is a schematic, sectional view of a first embodiment of a 
radiation-dose indicator in accordance with the invention. This 
radiation-dose indicator comprises an optically active layer 1 which is 
provided on a substrate 2. In the embodiment shown, the optically active 
layer is provided with a protective coating 3 to preclude damage to the 
optically active layer. However, said protective coating 3 is not 
necessary for the proper functioning of the indicator in accordance with 
the invention. 
For the substrate use can be made of glass or another inorganic material. 
It is alternatively possible, however, to use a flexible synthetic resin 
for the substrate. This is preferred, in particular if the indicator must 
be provided on an uneven substrate. If the radiation to be measured is 
directed via the substrate to the optically active layer then the 
substrate must be transparent to this radiation. It is noted that the 
presence of the substrate is not necessary for the proper functioning of 
the indicator in accordance with the invention. This applies in particular 
if the optically active layer is strong enough to be used as a 
self-supporting layer. In this case, after its manufacture, the optical 
layer is removed from the substrate. 
The optically active layer comprises a material having a liquid-crystalline 
order. Said liquid-crystalline material may exhibit nematic, cholesteric 
or smectic properties. Owing to this liquid-crystalline order the layer is 
opaque. In the absence of a dye in the layer, it will have a whitish 
(opaque) appearance. The layer also comprises a radiation-sensitive 
compound which breaks the liquid-crystalline order after the conversion 
process. This causes the liquid-crystalline material to become isotropic 
and hence transparent. 
The radiation-dose indicator in accordance with FIG. 1-A was manufactured 
as follows. Two transparent synthetic resin foils of polyethylene 
terephthalate were positioned at a short distance from each other by means 
of spacers. Subsequently, a thin, optically active layer was provided 
between the foils by means of capillary filling. This layer was composed 
of a mixture of 50 wt. % tolane in accordance with formula 1 of FIG. 5 and 
50 wt. % of the cyanobiphenyl compound in accordance with formula 5 of 
FIG. 5. The first foil serves as the substrate. The second foil serves as 
a covering layer to preclude evaporation of the liquid-crystalline 
molecules. The indicator thus obtained changed after an accumulated 
radiation dose of approximately 2.10.sup.4 J/cm.sup.2. 
FIG. 1-B shows a second embodiment of the indicator in accordance with the 
invention. This indicator comprises an optically active layer 1, a 
substrate 2 of glass and a protective coating 3 consisting of a glued-on 
synthetic resin foil. A small quantity of a dye is provided in the 
optically active layer. A fine lattice structure 4 of red colored material 
is provided between the optically active layer and the substrate. The 
apertures in the lattice serve to guide radiation to the optically active 
layer via the substrate. In the anisotropic phase, the color of the dye 
predominates in the indicator. After the radiation-sensitive conversion 
has taken place to a sufficient degree, this color disappears and the red 
color of the lattice becomes predominant. 
The indicator shown in FIG. 1-B was manufactured as follows. A glass 
substrate was provided over a surface area of 5 by 20 mm with a 5 
micrometers thick layer in the desired lattice structure by means of a 
printing technique. The material of the lattice was composed of a 
bisphenol-A/amine-epoxy system which cures at room temperature and in 
which 5 wt. % of a red pigment were dispersed. The adhesion to glass 
proved to be satisfactory. In addition, the lattice did not flow during 
the curing operation. 
An optically active layer was applied to the lattice. This layer was 
composed of a mixture of 50 wt. % of a bisphenol A epoxy, namely EPO-Tek 
301-2 available commercially from Epoxy Technology Inc.), 35 wt. % of the 
liquid-crystalline mixture E7 available commercially from Epoxy, and 15 
wt. % of the trans-cyanostilbene in accordance with the structural formula 
8 shown in FIG. 5. This layer was spread as a wet film on the substrate 
with the lattice. Subsequently, the optically active layer was cured by 
subjecting it to a temperature of 80.degree. C. for 3 hours or to room 
temperature for 3 days. Finally, a thin transparent coating was applied to 
the optical layer to protect said optical layer against contact. The 
indicator thus manufactured turned from white to red after it had been 
exposed to radiation having an energy of 5400 J/cm.sup.2. 
A third embodiment of the indicator in accordance with the invention was 
produced as follows. An optically active layer was spread from the melt on 
a flexible polyester substrate. This optically active layer was composed 
of the copolymer referred to as structure 9 in FIG. 5. This copolymer 
comprises the same active constituents as the optically active layer 
described in the second embodiment. The layer described in this 
embodiment, however, has a higher mechanical stability and is less subject 
to ageing. To obtain an optimally functioning indicator, the ratio of m:n 
must be approximately 1:5. In view of the processing of the polymer, the 
degree of polymerization (m+n) ranges between 5 and 100. Cyclic siloxanes 
can alternatively be used instead of linear polymers. Under these 
conditions the degree of polymerization must range between 3 and 7 in 
order to combine a low degree of evaporation of the liquid-crystalline 
materials with a sufficiently rapid switching time. 
A fourth embodiment of the indicator in accordance with the invention was 
produced as follows. The copolymer described in embodiment 3 was dispersed 
in isotropic bisphenol-A epoxy. The starting material used was composed of 
50 wt. % of the copolymer and 50 wt. % of the isotropic polymer. A 
quantity of 2 wt. % of the blue dye M-843 AQ (available commercially from 
Mitsui Toats) was added to this mixture. The "polymer dispersed liquid 
crystal polymer" (PDLCP) thus obtained was spread on a red substrate and 
cured. Owing to the liquid-crystalline order of the copolymer the 
indicator had a blue appearance. After prolonged exposure of the indicator 
to radiation having a wavelength of 360 nm, the liquid-crystalline order 
was broken and said indicator assumed the color of the underlying, 
contrasting red substrate. 
FIG. 2 shows a graph in which the transmission (T in %) of the inventive 
indicator in accordance with the fourth embodiment is plotted as a 
function of time. The transmission was measured at 450 nm. During the 
first 450 hours the transmission remains stable at a low level 
(approximately 5%). Subsequently, a rapid increase to the transmission to 
a maximum value of approximately 85% occurs. In reality, a color change 
from blue to red took place. 
FIG. 3 shows two lamps which are provided with an indicator in accordance 
with the invention. The lamp shown in FIG. 3-A is a gas-discharge lamp. 
The lamp shown in FIG. 3-B is an incandescent lamp. The lamp of FIG. 3-A 
comprises a tubular envelope 11 which is provided at the ends with 
closures 12 with electrical connection contacts 13. A radiation-dose 
indicator 14 is provided on the envelope. The color and/or transparency of 
this indicator changes after a certain number of burning hours of this 
gas-discharge lamp. The indicator is very advantageously used in discharge 
lamps emitting UV-light. 
FIG. 3-B shows an incandescent lamp comprising a glass envelope 21, a 
filament 22 and an electrical connection 23. A radiation-dose indicator 24 
is provided on the envelope. The color and/or transparency of this 
indicator changes after a certain number of burning hours of the lamp. The 
indicator is very advantageously used in incandescent lamps emitting IR 
light. 
FIG. 4 is a partly cutaway view of a tanning apparatus comprising a 
radiation-dose indicator. Said apparatus comprises a box-shaped housing 31 
which is provided at a main surface with a cover plate 32. This plate is 
transparent to ultraviolet radiation of a wavelength above 315 nm (UV-A 
radiation). This radiation is generated by a number of parallel, tubular 
low-pressure mercury discharge lamps 33. The cover plate 32 is provided 
with an exchangeable radiation-dose indicator 34. After a number of 
operating hours of the tanning apparatus, the color and/or transparency of 
the indicator changes. This indicates that a certain amount of radiation 
has been emitted. This change may indicate that a user has received, 
during one or more periods of use, a certain amount of radiation. Said 
change may alternatively indicate, however, that the lamps of the 
apparatus must be replaced. 
The present invention provides a reliable and accurate radiation-dose 
indicator which is relatively cheap to manufacture. The indicator is based 
on an optically active layer which comprises liquid-crystalline material 
in combination with a radiation-sensitive compound. The conversion product 
of the radiation-sensitive compound changes the order of the 
liquid-crystalline material. This change can be visually observed. The 
inventive indicator can very advantageously be used in UV and IR lamps as 
well as in tanning apparatus.