Disclosed is a nonglaring mirror which is an electrochromic device in principle. The mirror comprises a transparent front substrate laid with a transparent first electrode film, a back substrate laid with a second electrode film which is opposite and spaced from the first electrode film, first and second electrochromic layers formed on the first and second electrode films, respectively, and an electrolyte liquid which fills up the space between the two substrates. A reflecting surface is provided by using a transparent sheet as the back substrate and coating the rear surface of the back substrate with a reflecting metal film, or by using a reflecting metal film as the second electrode film. One of the first and second electrochromic layers is formed of a polymer of a conjugated compound, such as substituted or unsubstituted triphenylamine, and the other is formed of a transition metal oxide such as WO.sub.3. In the initial state both the first and second electrochromic layers are colorless and transparent. For coloration, the conjugated polymer layer is oxidized and the transition metal oxide layer is reduced. In the colored state the reflectance of the mirror becomes 7-15% and is controllable.

BACKGROUND OF THE INVENTION 
This invention relates to a nonglaring mirror, i.e. variable reflectance 
mirror, which is functional as an electrochromic device 
Known applications of liquid crystals include nonglaring mirrors That is, a 
liquid crystal cell having a transparent electrode film on the front side 
and a reflecting electrode film on the opposite side serves as a 
nonglaring mirror since reflectance of the mirror can be varied by 
application of an external electric field to change the state of the 
liquid crystal confined in a space between the two electrodes to thereby 
vary the light transmittance of the liquid crystal layer. However, 
practical use of liquid crystal mirrors is limited because of some 
disadvantages almost inherent to liquid crystals. In the case of a mirror 
using the light scattering effect of one type of liquid crystal, 
visibility of the mirror is significantly marred by bleeding of the 
reflected image when light transmittance of the liquid crystal is lowered. 
In the case of a mirror using the phase transition effect of another type 
of liquid crystal it is difficult to sufficiently lower the reflectance. 
Recently much effort has been devoted to research and development of 
nonglaring mirrors using electrochromic effects. A nonglaring mirror is 
obtained by providing a highly reflecting surface to an electrochromic 
cell of the transmissive type. Early proposals include the use of a 
solution of an organic electrochromic material represented by viologen as 
the transmittance controlling material confined between a transparent 
electrode and a reflecting electrode. However, such devices have problems 
with operating temperatures and service life. Also it is well known to 
replace the aforementioned electrochromic solution by a combination of a 
film of a transient metal oxide, such as WO.sub.3 which colors blue in a 
reduced state, deposited on the transparent electrode and an electrolyte 
solution confined between the two electrodes. However, there is still a 
problem with the durability of the mirror because it is likely that a 
portion of the electrolyte solution undergoes an irreversible 
decomposition reaction at the surface of the electrode opposite the 
electrochromic oxide film. 
In recently proposed electrochromic display devices which seem to be useful 
as nonglaring mirrors by the provision of a reflecting surface, it is 
common to coat the oppositely arranged two transparent electrode films 
with two different kinds of electrochromic materials, respectively. Also 
in these cases, the space between the two electrodes is filled with an 
electrolyte liquid. Typical combinations of two kinds of electrochromic 
materials are as follows. 
According to Japanese patent application primary publication No. 55-64216 
(1980), the first electrochromic material is a transition metal oxide 
which colors in a reduced state, such as WO.sub.3, and the second is a 
transition metal hydroxide which colors in an oxidized state, such as 
Ir(OH).sub.x. A disadvantage of a device or mirror using these 
electrochromatic materials is that coloring of the mirror is not deep 
enough because the transition metal hydroxide colors only palely so that 
the deepness of the mirror color is nearly equivalent to that of the 
transition metal oxide only. 
According to Japanese patent application primary publication No. 59-155833 
(1984), the first electrochromic material is a metal hexacyanometalate 
which is represented by M.sub.x [M'(CN).sub.6 ].sub.y (wherein M and M' 
are transition metals) and colors in an oxidized state, such as Prussian 
blue, and the second is a transition metal oxide which colors in a reduced 
state, such as WO.sub.3. There is a problem particular to any device using 
this combination of electrochromic materials. In producing the device it 
is inevitable that both the metal hexacyanometalate layer and the 
transition metal oxide layer are obtained in an oxidized state. That is, 
the former is in a colored state whereas the latter is in a bleached 
state. Therefore, it is necessary to perform an electrochemical treatment 
to reduce one of the two electrochromic layers before using the device. It 
is often occurs that the initial reduction treatment causes partial 
decomposition of moisture contained in its electrolyte liquid and 
oxidation of the electrochromic material in reduced state by the liberated 
oxygen. This is detrimental to the memory capability of the device. 
Besides, the decomposition of moisture is accompanied by some bubbling, 
which mars the appearance of the device. As a solution to this problem, 
Japanese patent application primary publication No. 59-159134 (1984) 
proposes to add an auxiliary electrode which comprises a reversibly 
oxidizable and reducible material. At tne initial reduction treatment the 
auxiliary electrode is used as the counter electrode. After that the 
auxiliary electrode serves no purpose. When the device is relatively small 
in size, the inutile space occupied by the auxiliary electrode becomes 
considerable compared with the effective coloring area. Besides, the 
provision of the auxiliary electrode raises the need of widening the 
distance between the two substrates coated with transparent electrode 
films and electrochromic layers. 
According to Japanese patent application No. 58-188518 (1983), the first 
electrochromic material is a metal hexacyanometalate, such as Prussian 
blue, and the second is either a conjugated polymer which becomes lower in 
light transmittance in a reduced state, such as polypyrrole, or a metal 
oxyhydroxide which colors in a reduced state, such as NiO(OH). Since the 
conjugated polymers used in this proposal are colored whether in an 
oxidized state or in a reduced state, the device using any of such 
polymers does not become colorless and transparent when bleached and 
therefore is not suitable for many uses where transparency is required of 
the device in a bleached state. When a metal oxyhydroxide is used it is 
difficult to form a sufficiently thick layer of the second electrochromic 
material, and deepness of coloring of the device is insufficient because 
of paleness of the color of the metal oxyhydroxide. 
For the above described reasons it is difficult to obtain fully practicable 
and sufficiently efficient nonglaring mirrors by using already developed 
or proposed electrochromic devices, while there is a strong demand for 
such nonglaring mirrors and particularly nonglaring rearview mirrors for 
automobiles. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electrochromic 
nonglaring mirror which is good in transprency in the bleached state and 
becomes sufficiently low in reflectance upon coloration and can be made 
compact so as to be useful as, for example, an automotive rearview mirror. 
The present invention provides an electrochromic nonglaring mirror, which 
comprises a transparent front substrate which is coated with a transparent 
first electrode film, a back substrate which is coated with a second 
electrode film and is held parallel to the front substrate such that the 
first and second electrode films are opposite to and spaced from each 
other, a first electrochromic layer formed on the first electrode film, a 
second electrochromic layer formed on the second electrode film, means for 
providing a reflecting surface to the back substrate and an electrolyte 
liquid in the space between the front and back substrates. This nonglaring 
mirror is principally characterized in that one of the first and second 
electrochromic layers is formed of a conjugated polymer which undergoes 
electrochromic oxidation and reduction and becomes lower in light 
transmittance when it is in an oxidized state and that the other 
electrochromic layer is formed of a transition metal oxide which becomes 
lower in light transmittance when it is in an electrochemically reduced 
state. 
Preferred examples of the conjugated polymer used in the present invention 
are polymers of substituted or unsubstituted triphenylamine, 
poly(para-phenylene), poly(N-methylpyrrole) and polyaniline. These 
conjugated polymers are colorless and transparent, or only palely colored, 
in electrochemically reduced state and are obtained in reduced state when 
formed as a coating film on an electrode. When oxidized, these polymers 
become very low in light transmittance presumably because of a change in 
the transitional absorption energy of .pi.-electron by double bond or 
unpaired electron. Among the above named conjugated polymers, 
polytriphenylamine and polymers of substituted triphenylamine are most 
favorable because these polymers become almost perfectly colorless and 
transparent when reduced and can be colored and bleached by a relatively 
low drive voltage. 
The electrochromic layer of a transition metal oxide such as WO.sub.3 is 
obtained in an oxidized state and, therefore, in a colorless and 
transparent state. Accordingly there is no need for an oxidizing or 
reducing treatment before using a mirror according to the invention. 
Naturally there is no need of incorporating an auxiliary electrode in the 
mirror. By application of a suitable voltage to oxidize the conjugated 
polymer layer and reduce the transition metal oxide layer, both of these 
two electrochromic layers assume color and become low in light 
transmittance, so that the reflectance of the mirror becomes sufficiently 
low. When the electrochromic layers have thicknesses suitable for 
possession of good resistance to peel, such as 500-1500 .ANG. in the case 
of a polytriphenylamine layer and 3000-5000 .ANG. in the case of a 
WO.sub.3 layer, the reflectance of the mirror in the colored state can be 
controlled to about 7-15%. Such values of lowered-reflectance are almost 
ideal for a nonglaring mirror. In conventional nonglaring mirrors on the 
principle of prism, lowered reflectance is about 4%. A nonglaring mirror 
of the invention can be made compact and is stable in the electrochromic 
effects. Accordingly this nonglaring mirror is useful for many purposes 
including rearview mirrors for automobiles. 
In a nonglaring mirror according to the invention, the reflecting surface 
is provided by coating the rear surface of the back substrate on condition 
that the back substrate is transparent, or by using a reflecting metal 
film as the second electrode film which is laid with one of the 
electrochromic materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the fundamental construction of a nonglaring mirror as a first 
embodiment of the invention. In principle, this mirror is an 
electrochromic device of the transmissive type. 
This device has front and back substrates 10 and 20 both of which are 
transparent. A transparent. electrode film 12 is deposited on the rear or 
inside surface of the front substrate 10, and a first electrochromic layer 
14 is formed, as a film on the electrode film 12. For this layer 14 the 
electrochromic material is a conjugated polymer, which undergoes 
electrochemical oxidation-reduction and becomes low in light transmittance 
when oxidized. Another transparent electrode film 22 is deposited on the 
inside surface of the back substrate 20, and a second electrochromic layer 
24 is formed, as a film, on this electrode film 22. The material of this 
electrochromic layer 24 is a transition metal oxide which assumes color 
and becomes low in light transmittance when electrochemically reduced. A 
lustrous metal film 26 is deposited on the outside surface of the back 
substrate 20 and serves as a means for providing a highly reflectrng 
surface necessary for a mirror. A transparent spacer 16 is used to keep a 
predetermined short distance between the first and second electrochromic 
layers 14 and 24. The spacer 16 is required to leave the electrochromic 
layers 14, 24 uncovered over as large an area as possible. In this 
embodiment the spacer 16 consists of a number of tiny glass spheres 
uniform in diameter. The two transparent substrates 10 and 20 are held 
spaced from each other by a thin layer 18 of a sealing material, which is 
applied peripherally of the substrates 10, 20 so as to surround the 
electrochromic layers 14, 24. The spaces defined between the two 
substrates 10 and 20 by the peripheral seal 18 is filled with an 
electrolyte liquid 30. 
Usually the transparent electrode films 12 and 22 are formed of SnO.sub.2 
or In.sub.2 O.sub.3. The transition metal oxide of the second 
electrochromic layer 24 can be selected from, for example, WO.sub.3, 
MoO.sub.3, Nb.sub.2 O.sub.5, Cr.sub.2 O.sub.3, Ta.sub.2 O.sub.5, 
TiO.sub.2, Fe.sub.2 O.sub.3 and AgO. In most cases WO.sub.3 is preferable. 
Conjugated polymers useful for the first electrochromic layer 14 are 
mentioned hereinbefore. It is preferred to use a polymer ootained by using 
triphenylamine or its derivative represented by the following general 
formula as the starting material. 
##STR1## 
wherein X, Y and Z each represent a hydrogen atom, a halogen atom, a 
hydroxyl group, an alkyl group, an alkoxyl group, an acyl group, an allyl 
group, a vinyl group or a vinylidene group. 
A film of such a polymer can be formed on the electrode 12 by using one of 
the following methods. 
(1) First a coating film of a selected monomer represented by the above 
general formula is formed on the electrode by a suitable method such as a 
solution coating method. Then the monomer in film form is polymerized by 
using a suitable oxidizer such as iodine, antimony pentafluoride, arsenic 
pentafluoride or ferric oxide to thereby accomplish fixing of a desired 
polymer film onto the electrode. 
(2) The selected monomer is polymerized by an ordinary polymerization 
method, and a solution of the obtained polymer is applied to the electrode 
so as to form a coating film. After that the polymer in film form is 
caused to undergo cross-linking reaction by using an oxidizer which can be 
selected from the above named ones. 
(3) The selected monomer or its polymer is dissolved in a suitable 
electrolyte liquid, and an electrolytic polymerization operation is 
carried out to thereby deposit a desired polymer film on the electrode. 
(4) This method can be employed when the selected monomer has an 
unsaturated hydrocarbon group such as allyl group or vinyl group as X, Y 
and/or Z in the above general formula. First, a film of the monomer is 
formed on the electrode by a suitable method such as a solution coating 
method. Then the monomer in film form is polymerized by heating or by 
irradiation with ultraviolet rays to thereby accomplish fixing of a 
desired polymer film to the electrode. 
The electrolyte liquid 30 is a solution of a supporting electrolyte, which 
is usually an alkali metal compound such as LiClO.sub.4, LiBF.sub.4, 
LiPF.sub.6, KClO.sub.4, KBF.sub.4 or KPF.sub.6, in an organic solvent such 
as acetonitrile, propylene carbonate or N,N'-dimethylformamide or in 
water, or in a mixed solvent which may contain water. 
The metal material of the reflective film 26 is not limited. It is possible 
to use any metal that provides a mirror surface. In practice it is 
convenient to use aluminum, silver or chromium. 
EXAMPLE 1 
A mirror of the construction shown in FIG. 1 was produced in the following 
manner. 
A glass sheet having a thickness of 1 mm was used as the transparent 
substrates 10 and 20. Each of the transparent electrode films 12 and 22 
was formed by vacuum deposition of SnO.sub.2 onto the substrate surface to 
a thickness of 3000 .ANG.. 
The material of the first electrochromic layer 14 was a polymer of 
4,4'-dichlorotriphenylamine, which was polymerized by a Grignard reaction 
and had an average molecular weight of about 2000. As to the 
polymerization method, reference is made to U.S. Pat. No. 4,565,860. The 
polymer was dissolved in chloroform in a concentration of 15 g/l, and the 
solution was applied to the surface of the transparent electrode film 12 
by a spin-coating method. After drying and degassing the polymer coating 
film, the front substrate 10 was placed in a vessel filled with iodine 
vapor and heated at 100.degree. C. for 2 hr to thereby accomplish 
cross-linking of the polymer on the transparent electrode 12. The thus 
formed first electrochromic layer 14 was a thin film having a thickness of 
about 1000 .ANG.. 
On the back side of the transparent substrate 20, the reflective metal 
coating film 26 was formed by sputtering of A1 to a film thickness of 
about 1500 .ANG.. After that the second electrochromic layer 24 was formed 
by vacuum deposition of WO.sub.3 onto the transparent electrode film 22 to 
a thickness of about 4000 .ANG.. 
The front substrate 10 was placed upside down, and a number of transparent 
glass spheres having a diameter of 40 .mu.m were distributed on the 
electrochromic layer 14 at a density of about 15 spheres per 
squarecentimeter. The glass spheres were used to constitute the spacer 16. 
To form the peripheral seal 18, an epoxy base adhesive was applied by 
screen printing to the marginal regions of the transparent electrode 22 on 
the back substrate 20 so as to leave an opening to be used for intake of 
the electrolyte liquid. Then the back substrate 20 was placed on the front 
substrate 10, and the adhesive used as the seal 18 was cured under an 
adequate pressure. The electrolyte liquid 30 was prepared by dissolving 1 
mole of LiClO.sub.4 in 1 liter of propylene carbonate to which about 3 
wt.% of water was added. The electrolyte liquid 30 was injected through 
the aforementioned opening into the space in the nearly completed mirror, 
and the opening was closed with the epoxy base adhesive. 
The film of the polymer of 4,4'-dichlorotriphenylamine formed as the first 
electrochromic layer 14 was initially colorless and transparent since the 
polymer, as formed, was in an electrochemically reduced state. Also, the 
film 24 of WO.sub.3 was initially colorless and transparent since WO.sub.3 
was in oxidized state. Therefore, the mirror of Example 1 exhibited high 
reflectance upon completion of the above described manufacturing process. 
Unlike a mirror using a combination of Prussian blue and WO.sub.3, the 
mirror of Example 1 does not need an initial reduction treatment and, 
hence, does not need to include an auxiliary electrode for such an initial 
treatment. In this case the reflectance of the mirror in the initial state 
was about 70%. 
The electrochromic function of the mirror of Example 1 was examined by 
voltametry. The electrode 22 coated with the WO.sub.3 film 24 was made the 
counter electrode, and the scan rate of the potential was 10 mV/sec. FIG. 
2 shows a cyclic voltamogram of the conjugated polymer film 14 on the 
front electrode 12. As can be seen, the mirror functioned as a good 
electrochromic device. In the case of an oxidation reaction with respect 
to the conjugated polymer film 14, the mirror colored dark blue at about 
0.9 V (vs WO.sub.3) so that the reflectance decreased to about 10%. In the 
case of a reduction reaction, the colorless and transparent state was 
resumed at about 0 V (vs WO.sub.3). 
The initial characteristics of this mirror as a nonglaring mirror using 
electrochromic effects were as follows. In the tested mirror samples, the 
effective mirror area was 50 cm.sup.2. 
With respect to the conjugated polymer film 14, the relationship between 
the quantity of the injected charge and the degree of coloration was as 
shown in FIG. 3, assuming that log(1/R), where R is reflectance, can be 
taken as the degree of coloration. Since the degree of coloration is 
proportional to the injected charge, it is apparent that the reflectance 
in the colored state can be further lowered by increasing the thickness of 
the conjugated polymer film 14 to thereby increase the quantity of 
injectable charge. Also it is apparaent that the reflectance can be set at 
a desired level by controlling the quantity of the injected charge. 
To evaluate the response time, the manner of change in reflectance at a 
constant electrode potential was measured for both oxidation potential and 
reduction potential. FIG. 4 shows the obtained results. In practice, it is 
permissible to assume that coloration or bleaching is complete when the 
amount of change in reflectance reaches about 90% of the maximum amount of 
change. It was found that the quantity of the injected or extracted charge 
required for such an extent of change in reflectance is about 4 
mC/cm.sup.2. A length of time required for injection or extraction of such 
a quantity of charge is taken as coloration time or bleaching time. The 
dependence of the coloration time and bleaching time on the electrode 
potential was measured to be as shown in FIG. 5. From FIG. 5 it is 
understood that the response time of this nonglaring mirror becomes about 
4 sec by applying a voltage of about 1.45 V (vs WO.sub.3)to the front 
electrode 12 for coloration and a voltage of about -0.35 V (vs WO.sub.3) 
for bleaching. In the colored state the reflectance was about 10%. 
The open-circuit memory capabilities were as shown in FIG. 6. When the 
electrochromic layers 14, 24 are left in Colored state, the reflectance 
slowly varies as time passes. The bleaching state is very stable with 
little change in reflectance. 
Reproducibility of the oxidation-reduction reaction was examined by 
repeating the sequence of keeping the conjugated polymer film 14 on the 
front electrode 12 at a coloring potential of 1.1 V (vs WO.sub.3) for 15 
sec and then at a bleaching potential of -0.4 V (vs WO.sub.3) for 90 sec. 
During repetition of this sequence of the quantity of charge injected at 
oxidation was measured at suitable intervals. As shown in FIG. 7, the 
oxidation-reduction reaction was stably reproducible so that a decrease in 
the quantity of the injected charge was almost negligibly small even when 
the sequential coloration and bleaching were repeated more than 30000 
times. 
EXAMPLE 2 
The mirror of Example 1 was modified only in that a polymer of 
4,4'-dibromo-4"-methyltriphenylamine, which was prepared by the method 
shown in U.S. Pat. No. 4,565,860 and had an average molecular weight of 
about 3000, was used as the material of the first electrochromic layer 14. 
The obtained mirror was comparable to the mirror of Example 1 in 
variability and controllability of reflectance. 
EXAMPLE 3 
The mirror of Example 1 was modified only in that a polymer of 
4,4'-dibromo-4"-methoxytriphenylamine, which was prepared by the method 
shown in U.S. Pat. No. 4,565,860 and had an average molecular weight of 
about 5000, was used as the material of the first electrochromic layer 14. 
The reflectance of the obtained mirror was variable within the range of 
from 65% to 15%. Therefore, this mirror too can be called a nonglaring 
mirror. 
EXAMPLE 4 
As the sole modification of the mirror of Example 1, the first 
electrochromic layer 14 was formed in the following manner. 
Triphenylamine was dissolved in chloroform in a concentration of 15 g/l, 
and the solution was spin-coated on the surface of the transparent 
electrode film 12. After drying and degassing the coating film, the front 
substrate 10 was placed in a vessel filled with iodine vapor and heated at 
100.degree. C. for 2 hr to thereby polymerize triphenylamine on the 
transparent electrode 12. Also in this case the thickness of the polymer 
film formed as the first electrochromic layer 14 was about 1000 .ANG.. 
The obtained mirror was comparable to the mirror of Example 1 in 
electrochromic functions and in variability and controllability of 
reflectance. 
FIG. 8 shows the fundamental construction of a nonglaring mirror as a 
second embodiment of the invention. In principle, this mirror does not 
differ from the mirror of FIG. 1. 
The distinction of the mirror of FIG. 8 resides in the manner of providing 
a highly reflecting surface to the back substrate 20. In this case the 
inside surface of the back substrate 20 is coated with a highly reflecting 
electrode film 23 instead of the transparent electrode film 22 in FIG. 1. 
Therefore. the back substrate 20 need not to be transparent, and the 
reflecting metal coating film 26 shown in FIG. 1 is omitted. Otherwise, 
the mirror or electrochromic device of FIG. 8 is identical with the mirror 
of FIG. 1 in both construction and materials. 
The material of the reflecting electrode film 23 is a metal that is 
electrochemically stable. It is preferred to use one of a group of metals 
whose atomic numbers are in the range from 73 to 79, i.e. tantalum, 
tungsten, rhenium, osmium, iridium, platinum and gold. If tungsten is 
used, it is easy to form an electrochromic layer of WO.sub.3 by oxidizing 
the surface of the reflecting electrode film. Among these metals, Ta, W, 
Pt and Au were each tested as an electrode material in an electrolytic 
cell. For comparison. Mo was tested similarly. The test was conducted in 
the following manner. 
Each of the above named five kinds of metals was deposited on a glass sheet 
by sputtering to thereby form a reflecting electrode film, and a film of 
WO.sub.3 was formed on each reflecting electrode film by sputtering. Each 
sample was immersed in an electrolyte liquid prepared by dissolving 1 mole 
of LiClO.sub.4 in 1 liter of propylene carbonate, and the voltage-current 
characteristic of the sample electrode was measured in a nitrogen gas 
atmophere by using a platinum wire as the counter electrode and a Ag/AgCl 
electrode as the reference electrode. The scan rate of the voltage at the 
sample electrode (vs Ag/AgCl) was 10 mV/sec. FIGS. 9-12 show cyclic 
voltamograms obtained by testing the WO.sub.3 -coated electrodes of Ta, W, 
Pt and Au. respectively. In every case the oxidation-reduction reaction of 
WO.sub.3 was accompanied by no side reaction attributed to the underlying 
metal film, and dissolution of the metal film into the electrolyte liquid 
did not take place. By this test, Ta, W, Pt and Au all proved to be good 
electrode materials. In the case of the sample of the Mo electrode film 
the cyclic voltamogram was as shown in FIG. 13. In this case some lowering 
of the rate of oxidation reaction was observed, and bleaching of the 
WO.sub.3 film remained incomplete. 
In the nonglaring mirror of FIG. 8 having the reflecting electrode 23, the 
distance between the opposite two electrodes 12 and 23 is a matter of 
importance. If the distance is inappropriate, ghost images or double 
images will appear when the electrochromic layers 14, 24 are colored so 
that reflection from the transparent electrode 12 becomes appreciable 
whereas the reflectance of the opposite electrode 23 becomes lower. For 
example, if the distance between the two electrodes is only about 10 .mu.m 
as is usual in conventional liquid crystal devices, incidence of 
homochromatic light such as the light from sodium lamps used in tunnels 
and service stations might cause appearance of ghost images in a striped 
pattern, which render the mirror unsightly. 
In general, reflection of light from two surfaces fixed at a distance D 
from each other results in interference of the reflected light waves. 
According to the Bragg's formula, the light waves reflected from the two 
surfaces intensify each other when the equation (1) is satisifed and 
weaken each other when the equation (2) is satisified. 
EQU 2D cos .theta.=n.lambda. (1) 
EQU 2D cos .theta.=(n+1/2).theta. (2) 
wherein .theta. is the angle of reflection, .lambda. is the wavelength of 
the incident light, and n is an integer. 
Since n is indefinite. there are many values of reflection angle .theta. 
that satisfy these equations. This is the reason for the appearance of 
so-called interference fringes. In a series of interference fringes which 
may appear in the mirror of FIG. 8, the interval between two adjacent 
fringes is determined if the wavelength .lambda. of the incident light, 
distance D between the two electrodes 12 and 23 and the distance L of the 
observer from the reflecting surface are given together with the value of 
a factor F which represents the degree of waviness of the reflecting 
surface. Assuming that .lambda. is 6000 .ANG., that L is 50 cm, that 
.theta. is 30.degree. and that errors in the distance D are 10% per length 
of 1 cm, the relationship between the distance D between the two 
electrodes 12 and 23 and the interval between the interference fringes was 
found by calculation. (If, for example, the distance D is 10 .mu.m and 
errors in D are as assumed above, the value of the aforementioned factor F 
is 1 .mu.m/1 cm.) The result of the calculation is represented by the 
curve A in FIG. 14. Under the assumed conditions the interference fringes 
become almost unnoticeable by the naked eye when the interval between the 
fringes is narrower than about 1 mm. Similar analysis was made also for 
the mirror of FIG. 1 wherein the reflecting surface (26) is on the outside 
of the back substrate 20. The thickness of the transparent substrate 20 
was assumed to be 1 mm and the angle of reflection to the observer to be 
about 5.degree.. That is, in this case the interference fringes are almost 
invisible. In a practical sense, even a homochromatic light contains 
wavelengths over a certain range. Therefore, the interference fringes 
overlap each other and become unnoticeable as the interval between the 
fringes narrows. 
From the above facts, it is understood that interference fringes in a 
mirror according to the invention can be rendered unnoticeable by 
sufficiently widening the distance between the front and back substrates 
10 and 20 or by forming the reflecting surface (26) on the outside surface 
of the transparent back substrate 20. In the case of using the reflecting 
electrode film 23 as shown in FIG. 8, it is important to appropriately 
determine the distance between the two substrates, i.e. distance D between 
the two electrodes 12 and 23. The gap between a ghost image and a true 
image also depends on the distance D, as represented by the curve C in 
FIG. 14. Under the conditions assumed hereinbefore, a ghost image in the 
mirror of FIG. 8 becomes almost unnoticeable by the naked eye when the gap 
between the ghost image and the true image is narrower than about 0.5 mm 
and becomes invisible when the gap is narrower than about 0.2 mm. In view 
of the relationship shown in FIG. 14. it is suitable to determine the 
distance D between the two electrodes 12 and 23 in the mirror of FIG. 8 so 
as to fall in the range of from 30 to 1000 .mu.m, and preferably in the 
range of from 30 to 504 .mu.m, for the purpose of rendering both 
interference fringes and ghost images unnoticeable. 
EXAMPLE 5 
A mirror of the construction shown in FIG. 8 was produced in the following 
manner. 
A glass sheet having a thickness of 1 mm was used as the front and back 
substrates 10 and 20. On the front substrate 10, the transparent electrode 
film 12 was formed by the same method as in Example 1. 
The material of the first electrochromic layer 14 was a polymer of 
unsubstituted triphenylamine polymerized by a Grignard reaction. The 
polymer was dissolved in chloroform in a concentration of 15 g/l, and the 
solution was spin-coated on the electrode film 12. In iodine vapor the 
coating film of the polymer was heated at 100.degree. C. for 2 hr to 
thereby fix a cross-linked polytriphenylamine film 14 to the transparent 
electrode film 12. The thickness of the polymer film 14 was about 1200 
.ANG.. 
On the back substrate 20, the reflecting electrode film 23 was formed by 
depositing Pt to a thickness of about 1000 .ANG. by sputtering. Then the 
second electrochromic layer 24 was formed by vacuum deposition of WO.sub.3 
onto the reflecting electrode film 23 to a thickness of about 4000 .ANG.. 
After that the assembly of the mirror including the spacer (glass spheres) 
16 and seal 18 was carried out in the same manner as in Example 1, and the 
electrolyte liquid 30 mentioned in Example 1 was introduced before 
completely sealing the assembly. 
In the obtained mirror, both the conjugated polymer film 14 and the 
WO.sub.3 film 24 were initially colorless and transparent. The reflectance 
of the mirror in the initial state was about 60%. This mirror functioned 
as a good electrochromic device and served as a nonglaring mirror. That 
is, the mirror colored dark blue when a voltage of 0.9 V (vs WO.sub.3) was 
applied to the electrode 12 coated with polytriphenylamine 14. In the 
colored state the reflectance of the mirror was about 7%. The mirror 
resumed the colorless and transparent state when the two electrodes 12 and 
23 Were short-circuited. 
By visual observation of the mirror surfaces, ghost images were almost 
unnoticeable. When the mirror was exposed to light from a sodium lamp the 
appearance of interference fringes was almost unnoticeable since the 
interval between the fringes was narrower than 1 mm. 
EXAMPLE 6 
As the sole modification of the mirror of Example 5, the first 
electrochromic layer 14 was formed in the same manner as in Example 1 by 
using a polymer of 4,4'-dichlorotriphenylamine. 
In the obtained mirror the reflectance was variable over the range of from 
about 50% to about 8%, and response of this mirror to driving voltages was 
better than that of the mirror of Example 5 by about 20%. 
In any of the foregoing examples, reversing of the two electrochromic 
layers by using WO.sub.3 as the first electrochromic layer 14 and a 
polymer of substituted or unsubstituted triphenylamine as the second 
electrochromic layer 24 resulted in no difference in the above described 
electrochromic functions of each mirror. 
FIGS. 15 and 16 show a rearview mirror unit for an automobile, in which an 
electrochromic nonglaring mirror according to the invention is used. The 
mirror 40 and a circuit board 44 are fitted in a mirror housing 42. 
Besides, a light sensor 46 for interior light and another light sensor 47 
for exterior light are fitted in the housing 42. Switches 50 for 
electrochromic functioning of the mirror 40 are disposed in a stay 52 
which supports the mirror housing 42. Numeral 48 indicates a pivot for 
adjusting the tilt of the mirror housing 42. 
FIG. 17 shows an example of the circuit installed in the mirror housing 42. 
Essentially. this circuit is comprised of a judgment circuit 58 which 
receives signals from the two light sensors 46 and 47 via two DC 
amplifiers 54 and 55, respectively, and determines an optimum value of the 
reflectance of the mirror 40, and a drive circuit 60 which applies a drive 
voltage to the mirror 40 in response to the output of the judgment circuit 
58.