Antifogging film and optical element using the same

In an optical reflection preventing film comprised of a plurality of layers provided on an optical element, one of the plurality of layers is formed by a transparent electrically conductive layer, and the optical film thickness of the transparent electrically conductive layer is 45-120 nm.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a transparent electrically conductive film whose 
transmittance is increased, and to an optical element in which said 
transparent electrically conductive film is caused to generate heat and 
thereby eliminate fogging occurring on the optical element. 
2. Related Background Art 
Cameras such as television cameras, video cameras and photographic cameras 
are sometimes used in an environment in which temperature and humidity 
change suddenly. 
For example, there is a case where photographing is carried out outdoors in 
the rainy season in which the rain on the previous day ceases and 
temperature rises due to the fine weather from the morning. Generally, if 
the temperature of the open air changes suddenly, the difference between 
the temperature in the camera and the temperature of the open air becomes 
great and fogging sometimes occurs on the lens surface of the photo-taking 
lens to make photographing impossible. 
Therefore, for example, in the case of a television camera or the like, 
heating means has heretofore been provided in the camera so that when the 
environmental conditions of the open air change suddenly, the temperature 
in the camera is increased by the heating means to thereby prevent fogging 
from occurring on the lens surface. 
However, it is very difficult to uniformly increase the temperature of each 
lens surface in the lens barrel and thereby prevent fogging from occurring 
on the entire lens surface, and it has often been the case that fogging 
inevitably occurs partly. 
Also, even in the case of an optical apparatus used indoors, fogging may 
sometimes occur on the imaging lens thereof to reduce the performance 
thereof. In Japanese Utility Model Publication No. 62-41383, it is 
proposed to provide an electrically conductive film on the objective lens 
of a pick-up device for digital disk, supply an electric current to this 
electrically conductive film through an electrode and thereby warm the 
objective lens. This is considered to be an application of the technique 
of preventing condensation from occurring on the window glass of an 
automobile. 
On the other hand, it has become popular to use a transparent electrically 
conductive film of indium oxide (In.sub.2 O.sub.3) or tin oxide 
(SnO.sub.2) as the transparent electrode of a liquid crystal panel. 
This transparent electrically conductive film, if it is a single piece, is 
not high enough in transmittance. 
As a method of decreasing the reflectance of a panel formed with a 
transparent electrically conductive film, there is known a film 
construction using a transparent electrically conductive film instead of a 
high refractive index layer, as a constituent film of a multilayer 
reflection preventing film formed on the surface of a substrate. 
For example, with .lambda..sub.0 as the central wavelength of the 
reflection preventing area, there is made a film construction comprising 
an integer layer (a layer having an optical film thickness integer times 
as great as 1/4 of .lambda..sub.0). 
(1) Substrate glass - In.sub.2 O.sub.3 (.lambda..sub.0 /2)-MgF.sub.2 
(.lambda..sub.0 /4) 
(2) Substrate glass - Al.sub.2 O.sub.3 (.lambda..sub.0 /4)-In.sub.2 O.sub.3 
(.lambda..sub.0 /2)-MgF.sub.2 (.lambda..sub.0 /4) 
where parentheses represent the optical film thickness obtained by 
multiplying the physical film thickness by the refractive index. 
However, with these constructions, the reflectance of the transparent 
electrically conductive film could be decreased, but the light absorption 
of the film itself could not be decreased. In the material forming the 
transparent electrically conductive film, electrons are ready to be 
excited from the value electron zone to the electrically conductive zone. 
Light energy is absorbed during this excitation and therefore, the 
transparent electrically conductive film is readier to absorb light than 
an ordinary optical film such as MgF.sub.2, ZrO.sub.2 or Al.sub.2 O.sub.3. 
In the reflection preventing film, this transparent electrically 
conductive film must be provided to a thickness corresponding to 
.lambda..sub.0 /2 for the central wavelength .lambda..sub.0. Where 
.lambda..sub.0 =500, a film of a refractive index 1.9 need be as thick as 
132 nm, and the decrease in transmittance by light absorption is great to 
a degree which cannot be neglected, and the use of such film, for example, 
as an antifogging film in an optical element whose transmittance is 
necessary up to approximately 100% leads to a great loss. 
SnO.sub.2 film (resistivity 2.times.10.sup.-3 .OMEGA..multidot.cm) which is 
a transparent electrically conductive film being presently used has a 
transmittance of only 85% for a film thickness of 125 nm, and likewise, 
In.sub.2 O.sub.3 film (resistivity 2.times.10.sup.-4 .OMEGA./cm) used as a 
transparent electrically conductive film provided a transmittance of only 
90% for a film thickness of 125 nm. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to increase the light 
transmittance of a multilayer film having a transparent electrically 
conductive layer. 
It is another object of the present invention to provide a device in which 
a transparent electrically conductive film provided on the optical element 
of an optical system is caused to generate heat and thereby prevent 
fogging from occurring.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some specific embodiments of the present invention will hereinafter be 
described with reference to the drawings. FIG. 1 shows one A of 
phototaking lenses for a television camera shown in FIG. 2. It is desired 
to provide an antifogging film on all lenses constituting the photo-taking 
lens, but when it is taken into consideration that it is difficult to 
provide wiring for supplying electric power to the antifogging film in a 
narrow gap in the lens barrel, the purpose will be sufficiently achieved 
if the antifogging film is provided on second and third lens surfaces on 
which condensation is most liable to occur. 
In FIG. 1, the reference numeral 1 designates an optical member whose outer 
peripheral portion comprises a circular lens of radius R. The reference 
numeral 2 denotes the transparent electrically conductive layer of a 
multilayer reflection preventing film. The transparent electrically 
conductive layer 2 is formed of indium oxide, tin oxide or the like as 
will be described in detail later, and is uniformly formed on the surface 
of the optical member 1. The reference numeral 3 indicates the effective 
diameter of the optical member 1. The reference numerals 4 and 5 designate 
electrodes each comprising an aluminum electrode terminal or the like for 
supplying electric power to the transparent electrically conductive layer 
2. The electrodes 4 and 5 and the transparent electrically conductive 
layer 2 together constitute a part of heat generating means. The 
electrodes 4 and 5 are of an arcuate shape having a predetermined 
dimension, and even number of such electrodes are disposed on the outer 
marginal portion of the effective circle 3 on the surface of the 
transparent electrically conductive layer 2. Of these electrodes, the 
electrodes 4 are, for example, positive electrodes, and the electrodes 5 
are negative electrodes. The reference numeral 6 denotes a DC stabilizing 
power source connected to the positive electrodes 4 and the negative 
electrodes 5. .omega. is the circumferential length of each arcuate 
electrode 4 (5), and is a dimension which satisfies 
##EQU1## 
where R is the radius of the outer peripheral portion, and which further 
satisfies 
EQU .omega..ltoreq.d.ltoreq.13.omega. (2) 
where d is the spacing between the adjacent electrodes. 
In the present embodiment, electric power is supplied to the transparent 
electrically conductive layer 2 by the use of the electrodes 4 and 5 
having the above-described dimension, whereby heat is uniformly generated 
on the surface in the effective circle 3 of the optical member 1. Thereby, 
fogging is effectively prevented from occurring on the surface of the 
optical member 1 when the outside environment changes suddenly. If the 
aforementioned conditions (1) and (2) are departed from, uniform heat 
generation will be come difficult. 
In the present embodiment, the width L of the electrodes 4 and 5 may be 
arbitrary if it is in the outer marginal portion of the effective circle 
3, and may preferably be 
##EQU2## 
when for example, the radius of the outer peripheral portion is R. 
The present invention will now be described by the use of specific examples 
of numerical values. As a first example, the diameter of the optical 
member 1 is 120 mm, the diameter of the transparent electrically 
conductive layer 2 is 106 mm, the effective diameter 3 is 100 mm, and 
electrodes having an arcuate length .omega. of 23 mm are provided 
equidistantly at four locations in the outer marginal portion of the 
effective diameter 3 so that the spacing d between the adjacent electrodes 
is 55 mm. 
When a voltage 10 V was applied from the DC stabilizing power source 6 to 
between the electrodes 4 and 5, the interior of the effective circle 3 of 
the film surface of the transparent electrically conductive layer 2 was 
heated to 30.degree.-54.degree. C. in about ten minutes thereafter, and no 
fogging occurred on the surface in the effective circle 3 of the optical 
member 1 even if humidity was rapidly over-saturated at an environmental 
temperature of 20.degree. C. In the present example, a good antifogging 
effect is obtained at a low power consumption of only 5 W. 
As a second example, an optical member having dimension similar to that in 
the first example was used, a pair of electrodes having a length .omega. 
of 80 mm were provided in opposed relationship with each other, a voltage 
12 V was applied from the DC stabilizing power source 6 to between the two 
electrodes, and an electric current of 0.5 A was supplied. In about 20 
minutes after the supply of the electric power, the surface in the 
effective circle 3 of the transparent electrically conductive layer 2 was 
uniformly heated in the range of 32.degree.-46.degree. C., and no fogging 
occurred on the surface in the effective circle 3 of the optical member 1 
even if humidity was rapidly over-saturated at an environmental 
temperature of 20.degree. C. In the present example, a good antifogging 
effect is obtained at a low power consumption of 6 W. 
The multilayer reflection preventing film will now be described. 
The reflection preventing film according to a specific example is 
constructed so that the film thickness of the transparent electrically 
conductive layer thereof is 120 nm or less and therefore, it is possible 
to reduce the light absorption of the transparent electrically conductive 
layer and obtain a sufficient transmittance for use in an optical element. 
Also, by adopting a film thickness in the range of 45-120 nm, the surface 
resistance becomes greater than in the prior art, and a sheet resistance 
appropriate for heat generation in an optical element such as an optical 
lens is obtained and thus, a reflection preventing film including a 
transparent electrically conductive layer which is good for use as an 
antifogging film becomes possible. 
Herein, optical film thicknesses .lambda./4 and .lambda./2 are general 
terms, and strictly, the film thickness deviates more or less from these 
values in conformity with the characteristic necessary for the optical 
element used. Specifically, the ranges of the film thicknesses are as 
follows: 
Film thickness .lambda./4: more than 0.18.lambda. and less than 
0.27.lambda. 
Film thickness .lambda./2: more than 0.39.lambda. and less than 0.6.lambda. 
Referring to FIG. 3 which schematically shows the construction of the 
reflection preventing film, the reference numeral 11 designates a layer of 
MgF.sub.2 having a low refractive index whose optical film thickness is 
.lambda..sub.0 /4 for the central wavelength .lambda..sub.0 of the 
reflection preventing region, and having a refractive index of 1.38. The 
reference numerals 12 and 13 denote high refractive index layers, i.e., 
equivalent films, the sum of whose optical film thicknesses is equivalent 
to .lambda..sub.0 /2 for the central wavelength .lambda..sub.0 of the 
reflection preventing region, and the layer 12 is a film of ZrO.sub.2 
having a higher refractive index than the transparent electrically 
conductive layer 13 and having a refractive index of 2.12, and the layer 
13 is a transparent electrically conductive layer of indium oxide 
containing tin oxide having a refractive index of 1.9 (hereinafter named 
generically as the ITO layer), and the details thereof will be described 
later. The reference numeral 14 designates an optical lens of diameter 100 
mm which is a substrate, the reference numeral 15 denote electrodes for 
supplying electric power to the transparent electrically conductive layer, 
and the reference numeral 16 designates a power source for supplying 
electric power to the transparent electrically conductive layer 13 to heat 
the surface of the optical lens and prevent condensation. 
When an electric current is supplied from the power source 16 through the 
electrodes 15 to the ITO layer 13 of the reflection preventing film 
provided on the optical lens 14, the ITO layer 13 generates heat and the 
heat is transferred to the MgF.sub.2 layer 11 which is the outermost layer 
of the reflection preventing film, whereby condensation on the surface of 
the MgF.sub.2 layer 11 is prevented. 
The transmission loss rates and sheet resistances of the ITO layer whose 
mixture weight ratio of SnO.sub.2 is 5% and which was made with the film 
thickness thereof varied for lights of wavelengths 400 nm, 450 nm and 650 
nm are shown in FIG. 9 (the broken line indicates the sheet resistance 
value). 
From this figure, it is seen that the transmission loss rate is of a low 
value in the visible light range if the film thickness is 120 nm or less. 
Also, the sheet resistance value rises sharply if the film thickness is 45 
nm or less, and sufficient heat generation is not obtained unless the 
voltage is increased. Accordingly, the film thickness of the ITO layer 
should effectively be set to the range of 45-120 nm. 
For example, the reflection preventing characteristic when the optical film 
thicknesses of the respective layers are 190 nm, 55 nm and 138 nm from the 
optical lens side for the central wavelength length .lambda.=552 nm as the 
film construction of the reflection preventing film of FIG. 3 and an 
optical lens of refractive index 1.52 is used as the substrate 14 is shown 
in FIG. 4. The then actual film thickness of the ITO layer is 100 nm. For 
comparison, the reflection preventing characteristic of a reflection 
preventing film in which the .lambda..sub.0 /2 layer, the ITO layer 13 and 
the ZrO.sub.2 layer 12 of FIG. 3 are replaced with an ITO layer of optical 
film thickness .lambda..sub.0 /2 is shown in FIG. 8. Comparing the two, it 
is seen that a sufficient reflection preventing characteristic is obtained 
by the construction of the present example in the range of 450-650 nm 
which is important in the optical lens. Also, in the film for comparison, 
the film thickness of the ITO layer is as great as 132 nm. In the present 
example, it is 100 nm. Since the light absorption amount of the ITO layer 
increases in conformity with the film thickness thereof, the present 
example is smaller in light absorption and a high transmittance is 
obtained. 
The film surface when an electric current of 0.5 A was actually supplied to 
an ITO layer of sheet resistance 20 .OMEGA./sq was heated to 40.degree. C. 
in about 10 minutes after the supply of the electric power, and no fogging 
was observed on the optical lens in a condition in which the environmental 
temperature was 25.degree. C. and humidity was over-saturated, and the 
antifogging effect could be confirmed. It should be noted that a 
satisfactory reflection preventing function required of the optical lens 
was maintained. 
Shown in FIG. 5 is the reflection preventing characteristic of a film 
construction in which, as a modification of the film construction of the 
reflection preventing film of FIG. 3, an ITO layer having an optical film 
thickness of 90 nm as a transparent electrically conductive layer and a 
ZrO.sub.2 layer of a higher refractive index 2.1 having an optical film 
thickness of 194 nm, for a light of the central wavelength .lambda..sub.0 
=476 nm, are formed on an optical lens 14 having a refractive index of 
1.52 and a diameter of 50 mm and a .lambda..sub.0 /2 layer is formed by 
these two layers and an MgF.sub.2 layer of a low refractive index 1.38 
having an optical film thickness of 119 nm which is the final layer is 
formed as a .lambda./4 layer. In this example, the thickness of the ITO 
layer is as small as 47 nm while the reflection preventing characteristic 
remains good. From the comparison of FIG. 5 with FIG. 4, it is seen that 
the reflection preventing characteristic is varied. The layer thickness of 
the ITO layer may be thus varied between 45 to 120 mm, whereby fine 
correction of the characteristic is possible without changing the basic 
construction, that is, without greatly varying the reflection preventing 
characteristic. 
When an electric current of 0.3 A was actually supplied from the 
stabilizing power source to the ITO layer, the surface of the layer was 
heated to 40.degree. C. in about 10 minutes after the supply of the 
electric current, and no fogging occurred on the optical lens even if 
humidity became over-saturated in a temperature atmosphere of 25.degree. 
C., and a satisfactory reflection preventing function was maintained. 
As a fifth example, the construction of a reflection preventing film in 
which, for the design wavelength .lambda..sub.0, the basic construction 
comprising integer layers is formed by three layers from a substrate, 
i.e., a layer of film thickness .lambda..sub.0 /4 having a refractive 
index higher than that of the substrate, a layer of film thickness 
.lambda..sub.0 /2 having a higher refractive index and a layer of film 
thickness .lambda..sub.0 /4 having a refractive index lower than that of 
the substrate and the first layer is replaced with a transparent 
electrically conductive film 23 comprising non-integer layers and a 
non-integer layer 25 having a lower refractive index and the second layer 
is replaced with a film comprising three non-integer layers including a 
non-integer layer 24 of a refractive index lower than that of two 
non-integer layers 22a and 22b of a high refractive index therebetween is 
schematically shown in FIG. 6. 
Specifically, for a light of the central wavelength .lambda..sub.0 =540 nm, 
the refractive index of the substrate was 1.70, and the refractive indices 
of the respective layers from the substrate side were: 1.9 for the ITO 
transparent electrically conductive layer, 1.6 for Al.sub.2 O.sub.3, 2.1 
for ZrO.sub.2, 1.6 for Al.sub.2 O.sub.3, 2.1 for ZrO.sub.2, and 1.38 for 
MgF.sub.2 ; the optical film thicknesses of the respective layers were 90 
nm, 20 nm, 159 nm, 23 nm, 66 nm and 135 nm; and the diameter of the 
substrate was 60 mm. The reflection preventing characteristic of this film 
is shown in FIG. 7. 
The ITO layer comprising non-integer layers is combined with a non-integer 
layer having the same medium refractive index to make them equivalent to 
an integer layer having a medium refractive index lower than the high 
refractive index layer, thereby obtaining a good reflection preventing 
characteristic without changing the optical film thickness. 
When an electric current of 0.35 A was supplied from the stabilizing power 
source to the ITO layer, the surface of the layer was heated to 40.degree. 
C. in about 10 minutes after the supply of the electric current, and no 
fogging occurred on the optical lens even if humidity became 
over-saturated in a temperature atmosphere of 25.degree. C., and an 
efficient antifogging function was exhibited at a power consumption of 8 W 
and a sufficient reflection preventing function was maintained. 
Description will hereinafter be made of the action of an ITO layer 
consisting of In.sub.2 O.sub.3 of a refractive index of 1.9 mixed with 
SnO.sub.2 of a total weight ratio 2.5-6%. 
Referring to FIG. 10 which is a schematic construction view, the reference 
numeral 31 designates an MgF.sub.2 layer of a refractive index of 1.38, 
the reference numeral 32 denotes a ZrO.sub.2 layer of a refractive index 
of 2.1, the reference numeral 33 designates an ITO layer, and the 
reference numeral 34 denotes an optical lens formed of a material of a 
refractive index of 1.52 which is a substrate, i.e., an optical element, 
having this multilayer film attached to the surface thereof. The 
refractive indices of the layer 31, 32 and 33 relative to the substrate 
are low, high and medium (lower than the layer 32 and higher than the 
substrate) in the named order. Also, the optical film thicknesses (the 
actual film thickness x refractive index) of the respective layers for the 
approximate central wavelength and reference wavelength .lambda. of the 
light which is the object of this optical element are .lambda./4 for the 
MgF.sub.2 layer 31 and .lambda./2 for the ZrO.sub.2 layer 32 and the ITO 
layer 33 as combined together. A reflection preventing film is formed by 
this multilayer construction. The reference numeral 15 designates 
electrodes for supplying electric power to the ITO layer 33, and the 
reference numeral 16 denotes a variable power source for applying a 
voltage to the ITO layer 33 through the electrodes 15 to thereby cause the 
ITO layer 33 to generate heat. The surface portion of the optical lens 14, 
i.e., the MgF.sub.2 layer 31, is heated by the proper supply of electric 
power from the variable power source 16, whereby condensation is 
prevented. 
Here, the mixture weight ratio of SnO.sub.2 which is an evaporating 
material was varied in the range of 0-10% and the transmission loss rates 
and sheet resistances of an ITO layer of film thickness 80 nm obtained by 
the vacuum evaporation method in lights of wavelengths 400 nm, 450 nm and 
650 nm were measured. The transmission loss rates and sheet resistance 
characteristic by the mixture weight ratio obtained thereby are shown in 
FIG. 11 (the broken line indicates the sheet resistance value). It is seen 
from this figure that when the weight ratio of SnO.sub.2 is 2.5-6%, 
particularly 3-5%, the transmission loss amount (the light absorption 
amount) becomes smallest. This is considered to be because by adding a 
small amount (2.5-6%) of SnO.sub.2 to In.sub.2 O.sub.3 , the exciting 
energy from the value electron zone to the conductive zone moves to the 
high energy side and therefore the absorbing area which was the short 
wavelength side of the visible range moves to the ultraviolet side. 
Accordingly, if a mixture layer consisting of In.sub.2 O.sub.3 
with SnO.sub.2 of a weight ratio 2.5-6%, particularly 3-5%, added thereto 
is a constituent layer of the reflection preventing film, there can be 
realized a reflection preventing film which is small in the absorption by 
the transparent electrically conductive layer. In this range, the sheet 
resistance value also is a low value, and sufficient heat generation is 
obtained even at a low voltage value. 
Further, the transmission loss rates and sheet resistances of an ITO layer 
made at a mixture weight ratio 5% of SnO.sub.2 with the layer thickness 
varied, for lights of wavelengths 400 nm, 450 nm and 650 nm, are shown in 
FIG. 9 (the broken line indicates the sheet resistance value). 
It is seen from this figure that if the layer thickness is about 200 nm or 
less, the transmission loss rate is generally of a low value in the 
visible light range. Also, for a layer thickness of about 40 nm or less, 
the sheet resistance value rises sharply and sufficient heat generation is 
not obtained unless the voltage is increased. Accordingly, the film 
thickness of the ITO layer should effectively be set to the range of 40 
nm-200 nm, particularly the range of about 45 nm-120 nm. 
The details of a multilayer film of the reflection preventing film 
construction of FIG. 10 will now be described as a specific example. A 
film of the aforedescribed construction is formed on the surface of an 
optical lens having a diameter of 100 mm. The ITO layer is deposited by 
vacuum evaporation by evacuating the interior of the bell jar of an 
evaporating apparatus to a degree of vacuum of 1.times.10.sup.-5 Torr or 
less, and thereafter introducing oxygen gas thereinto and regulating the 
interior thereof to a degree of vacuum of 4.0.times.10.sup.-4 Torr, and 
heating a mixture evaporating material of In.sub.2 O.sub.3 and SnO.sub.2 
(the total weight ratio of SnO.sub.2 is 5%) by an electron beam, and 
fluctuating the evaporation speed on a glass substrate heated to a 
temperature of 300.degree. C. in the range of 1-3.ANG./sec. in accordance 
with the progress of film formation. The film thicknesses of the 
respective layers are: 100 nm for the MgF.sub.2 layer, 26 nm for the 
ZrO.sub.2 layer, and 100 nm for the ITO layer. If the reference wavelength 
.lambda.=550 nm, the film thickness of the MgF.sub.2 layer corresponds to 
.lambda./4, and the film thickness of the ZrO.sub.2 layer and the film 
thickness of the ITO layer compositely correspond to .lambda./2. The 
result obtained by applying a visible light and measuring the reflection 
preventing characteristic is as shown in FIG. 4. A reflection preventing 
film having a sufficient reflection preventing characteristic can be made 
by thus using an ITO layer of the aforedescribed construction. From this, 
it is seen that a reflection preventing film small in both reflection and 
absorption, that is, very high in transmittance, has become by the 
construction of the present invention. In fact, when an electric current 
of 0.5 A was supplied to the ITO layer in this film at a DC voltage of 12 
V, the surface of the layer was heated to 40.degree. C. in about 10 
minutes after the supply of the electric current, and no fogging was seen 
on the optical glass even if the humidity was rapidly over-saturated at an 
environmental temperature of 25.degree. C., and it could be confirmed that 
the resultant film was a reflection preventing film having an antifogging 
action at a low power consumption of only 6 W. Thus, it is seen that the 
characteristic as an antifogging film also is good. 
The reflection preventing film according to another example of the present 
invention in which for the reference wavelength .lambda., a film thickness 
corresponding to (1/4-1/2-174 ).lambda. is the basic film construction is 
shown in FIG. 12. 
In FIG. 12, members similar to those in FIG. 10 are given similar reference 
numerals. The reference characters 47a and 47b designate ZrO.sub.2 layers 
of a refractive index of 2.1, and the reference numeral 48 denotes an 
Al.sub.2 O.sub.3 layer of a refractive index of 1.63. A .lambda./2 layer 
is formed by the layers 47a, 48 and 47b. The present example will be 
described in detail as a specific example. The film of FIG. 12 is made by 
a method similar to the method of the example of FIG. 10. On an optical 
lens formed of a material of a refractive index of 1.5 for a light of a 
reference wavelength .lambda.=480 nm, an ITO layer having a refractive 
index of 1.9 and a film thickness 54 nm and a weight ratio 5% of SnO.sub.2 
was constructed as a layer having a film thickness corresponding to 1/4 
.lambda., a ZrO.sub.2 layer having a refractive index of 2.1 and a film 
thickness of 61 nm, an Al.sub.2 O.sub.3 layer having a refractive index of 
1.63 and a film thickness of 12 nm and a ZrO.sub.2 layer having a 
refractive index of 2.1 and a film thickness of 17 nm were constructed as 
a layer having a film thickness corresponding to 1/2 .lambda., and an 
MgF.sub.2 layer having a refractive index of 1.38 and a film thickness of 
86 nm was constructed as a layer having a film thickness corresponding to 
1/4 .lambda.. The reflection preventing characteristic of this reflection 
preventing film is shown in FIG. 13. Thus, in such other multilayer film 
construction as well, the reflection preventing characteristic can be made 
good and a high transmittance can be realized together with a low 
absorbing property. 
When the film construction of such reflection preventing characteristic was 
adopted for optical glass of 16 mm square and an electric current of 0.1 A 
was supplied to the ITO layer at a voltage of 6 V, the surface of the 
layer was heated to 40.degree. C. in about 10 minutes after the supply of 
the electric current, and no fogging was seen on the optical glass even if 
the humidity was rapidly over-saturated at an environmental temperature of 
25.degree. C., and it could be confirmed that this film was a reflection 
preventing film having an antifogging action at a low power consumption of 
only 0.6 W. The characteristic as an antifogging film also is good.