United States Patent
`Noguchi
`
`19)
`
`ACTAAAAA
`
`US005334916A
`(11) Patent Number:
`[45] Date of Patent:
`
`©
`
`5,334,916
`Aug. 2, 1994
`
`[54] APPARATUS AND METHOD FOR LED
`EMISSION SPECTRUM CONTROL
`
`Inventor: Masahiro Noguchi, Ibaraki, Japan
`(75]
`[73] Assignee: Mitsubishi Kasei Corporation,
`Tokyo, Japan
`[21] Appl. No.: 888,758
`
`May 27, 1992
`[22] Filed:
`[30]
`Foreign Application Priority Data
`May 27,1991 [JP]
`Japan oo...cesececesesseeereeeee 3-121317
`
`Int. C15 oe GOS5F 1/00; HOSB 41/36
`(51)
`[52] US. Che occ ccececsseecsessescsssseessereessnssacees 315/309
`[58] Field of Search ...........ccccccecseeees 315/291, 309
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Primary Examiner—Tony M. Argenbright
`Attorney, Agent, or Firm—Armstrong, Westerman,
`Hattori, McLeland & Naughton
`
`[57]
`ABSTRACT
`An emission spectrum controlling apparatus and
`method for an LEDare disclosed. A temperature mea-
`surement device is provided for measuring the tempera-
`ture of a light emitting diode or the temperature in the
`surrounding environmentofthe light emitting diode. A
`driving power control device is also provided for con-
`trolling the driving power of the LED,and a comput-
`ing unit is disclosed which controls the driving power
`control device based on the temperature information
`from the temperature measuring device and the driving
`power information from the driving power control
`device.
`
`4,529,949
`
`7/1985 deWit et al... 330/289
`
`8 Claims, 3 Drawing Sheets
`
`
`
`
`
`LED APPLIED VOLTAGE -
`MEASUREMENT AND
`CONTROL MEANS
`
`LED DRIVING
`CURRENT
`MEASUREMENT
`AND CONTROL
`MEANS
`
`
`
`LED TEMPERATURE
`MEASUREMENT
`
`
`MEANS
`
`
`
`
`
`
`EMISSION WAVELENGTH
`CONTROL MEANS
`
`
`COMPUTING UNIT
`
`
`1
`
`APPLE 1008
`
`APPLE 1008
`
`1
`
`

`

`U.S. Patent
`
`Aug. 2, 1994
`
`Sheet 1 of 3
`
`5,334,916
`
`
`
`LED APPLIED VOLTAGE
`MEASUREMENT AND
`CONTROL MEANS
`
`
` LED DRIVING
`LED TEMPERATURE
`MEASUREMENT
`CURRENT
`
`MEASUREMENT
`MEANS
`
`
`
`AND CONTROL
`MEANS
`
`
`
`
`EMISSION WAVE LENGTH
`CONTROL MEANS
`
`
`
`COMPUTING UNIT
`
`2
`
`

`

`U.S. Patent
`
`Aug. 2, 1994
`
`Sheet 2 of 3
`
`5,334,916
`
`SI. 32
`B =4.707x 10E-4
`
`
`TEMPERATURE
`INPUT
`
`Fig. 3
`
`
`
` miz=axixv
`
`m2= Eg-60x8
`
`HC1=1240/(m2- m1)
`
`
`
`
`
`3
`
`

`

`U.S. Patent
`
`Aug. 2, 1994
`
`‘Sheet 3 of 3
`
`5,334,916
`
`WAVELENGTH, HC2
`
`
` INPUT OF A PRESENT
`WAVELENGTH, HC!
`
` INPUT OF A DESIRED
`4!
`
`
`
`
`
`DECREASE
`CURRENT
`
`INCREASE
`THE
`CURRENT
`
`Fig.4
`
`4
`
`

`

`2
`temperature of an LED or the surrounding ambient
`temperature and the standard temperature. The temper-
`ature of the LED itself or the surrounding ambient
`temperature and the driving power of the LED are
`detected. Then, the emission wavelength energy can be
`calculated by subtracting the value of an applied power
`multiple by a specified coefficient and the difference
`from the standard temperature multiple by a specified
`coefficient from the optical band gap at the standard
`temperature. The emission wavelength can be con-
`trolled by controlling the driving power of the LED
`based on the output of the computing unit.
`In FIG. 1, 1 is an LED with a sensor for measuring
`temperature, 2 is a measurement and control meansfor
`an LEDdriving current, 3 is a measurement and control
`means for an LED applied voltage, 4 is a measurement
`means for an LED temperature, 5 is a computing unit,
`and 6 is a control means for an emission wavelength.
`The temperature of the LED is measured by a tem-
`perature sensor. However, the temperature to be mea-
`sured is not limited to the temperature of the LED
`itself, but the temperature in the environment surround-
`ing the LED can also be measured. The sensor to be
`used can be either a contact-type or a non contact-type
`An emission spectrum could be quantitatively and
`sensor. The desirable range in which the sensoris to be
`precisely monitored by using a spectrometer type mea-
`disposed is within a radius of 300 mm fromaradiating
`suring system for monitoring the emission spectrum,the
`system incorporating large and expensive devices such
`LED,if a contact-type sensoris used, and within 15 mm
`that the system as a whole is expensive and difficult to
`is a non contact-type sensoris used.
`handle.
`The number of LEDsor sensors used in the present
`invention can be more than one each. If the shortest
`One object of the present invention is to improve the
`above-noted problems of expense andsize.
`distance among the various positions of sensors and
`Another object of the present invention is to provide
`LEDsis within the range of 300 mm (or 15 mm for
`an emission spectrum control apparatus and method
`noncontact sensors), the arrangement will be suitable
`which is small sized, easy to handle, has a simplified
`for achieving good results in the present invention. A
`form, and which also has a low manufacturing cost by,
`thermocouple or a platinum resistor, as examples, can
`for example, developing integrated circuitry.
`be used as the contact-type sensor in the present inven-
`In accordance with the above-noted objects, an emis-
`tion, and an infrared ray detector or a thermopile, as
`sion spectrum control apparatus and method is pro-
`examples, can be used as the noncontact-type sensor in
`vided comprising a temperature measurement meansfor
`the present invention.
`measuring the temperature of an LED orfor measuring
`In FIG.2, 11 is an LED, 12 is a C-A thermocouple,
`the temperature in the environment in which the LED
`13 is an epoxy adhesive, 14 is a Au wire, 15 is a transpar-
`is disposed, a driving power control meansfor control-
`ent coating resin, 16 is a terminal pin and 17is a two pin
`ling the driving power of the LED, and a computing
`header. In the embodimentillustrated in FIG. 2, LED
`unit which controls the driving power control means
`11 is bonded to a can type header 17, and the C-A ther-
`based on temperature information from the temperature
`mocouple 12 is fixed with an epoxy adhesive 13 (e.g.
`measurement means and driving power information
`“Araldite”). The LED and terminal pin 16 are con-
`from the driving power contro! means. A plurality of
`nected with a Au wire 14, and this arrangement is
`LEDs and a plurality of temperature measurement
`coated with the transparent coating resin 15.
`means can be utilized in the present invention.
`In operation, a temperature signal detected by LED
`lamp 11 is converted to a digital signal by the LED
`temperature measurement means 4 having a voltmeter
`and an A/D converter. The temperature informationis
`input to computing unit 5 along with information from
`the measurement and control means for the LED driv-
`ing current 2 and the measurement and control means
`for the LED applied voltage 3.
`In the computing unit 5, a computing operation is
`performed in order to determine the emission wave-
`length energy based on the equation:
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`35
`
`EWE=OB~—aXxAP—8X DST,
`
`()
`
`65
`
`wherein EWEis the emission wavelength energy,
`OBis the optical band gap at a standard temperature,
`APis the applied power, and DST is the difference
`from a standard temperature.
`
`1
`
`5,334,916
`
`APPARATUS AND METHODFORLED EMISSION
`SPECTRUM CONTROL
`
`BACKGROUNDOF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to an apparatus and
`method for controlling the emission spectrum of an
`LEDwith highprecision, and can be utilized in connec-
`tion with an emission element (LED) standard light
`source, an LEDlight source for a sensor, optical com-
`munication equipment,etc.
`.
`2. Description of the Related Art
`It is known in theory that an emission spectrum of an
`LED changes along with ambient temperature or a
`driving current. When an LEDstandardlight sourceis
`manufactured which emits light of a particular spec-
`trum, for example, based on the knowledgein theory of
`changes of emission spectrum of an LED, the LED is
`set in a thermostatic chamber and the temperature of
`the LEDis kept at a specified value in monitoring the
`emission spectrum.
`
`SUMMARYOF THE INVENTION
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will be further explained in
`connection with the accompanying drawings wherein:
`FIG. 1 is a block diagram showing the system for
`LEDemission spectrum control;
`FIG.2 illustrates an embodiment of the present in-
`vention having an LED with a thermocouple;
`FIG. 3 is a flow chart for illustrating the system for
`calculating an emission wavelength; and
`FIG. 4 is a flow chart illustrating the process for
`controlling the emission wavelength.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`invention, an emission wavelength
`In the present
`varies according to an optical band gap at a standard
`temperature, driving power and difference between the
`
`5
`
`

`

`5,334,916
`
`3
`The standard temperature from equation (1) can be an
`arbitrary temperature predetermined in advance, and
`the coefficients a and £ are values to be obtained exper-
`imentally based on the particular material, shape, etc.
`constituting the LED. The optical band gap of a mate-
`rial is decreased by the heat generated by the driving
`power or a temperature rise. Since the wavelength
`energy (a reciprocal of an emission wavelength)
`changes along with the change of the optical band gap,
`the values of the coefficients a and £ are found experi-
`mentally, and the emission wavelength is obtained from
`equation (1). Thus, the emission wavelength spectrum
`can be controlled by controlling the driving power of
`the LED.
`Based on the calculation result from equation (1) in
`the computing unit, the difference between the calcula-
`tion result and a desired wavelength is found in the
`emission wavelength control means 6. The emission
`wavelength is controlled based on equation (1) by con-
`trolling the current measurement and control means 2
`and/or voltage measurement and control means 3. The
`powercontrol for an LED can be performed by con-
`trolling current or voltage individually or both simulta-
`neously. Thus, a desired emission wavelength spectrum
`can be obtained from the LED.
`FIG.3 illustrates the emission wavelength calcula-
`tion process of the present invention. As can be seen in
`FIG. 3, a=0.1946 and 8=4.707 x 10~4 and an optical
`gap EG,at a standard temperature = 1.828580 eV (steps
`31 to 33). Then, 0.03 A, 1.86 V and 60° C.are substi-
`tuted for a currenti, a voltage v, and a temperature T,
`respectively (steps 34-36). In step 37, m1=a XixXv, and
`m2=EG-— 60 are determined. In step 38, a present
`emission wavelength HC1
`is
`obtained where
`HC1= 1240/(m2—~—m1). It should be noted that m2—m1
`correspondsto the right side of equation (1), the recip-
`rocal of which is the emission wavelength (1240 in step
`38 is a constant).
`in the emission wavelength
`As shown in FIG. 4,
`control process of the present
`invention, a desired
`wavelength HC2is at first input and then the present
`wavelength HC1is input (steps 41 and 42). Then, HC1
`and HC2 are compared.If HC1is larger than HC2(step
`43), the current is decreased (step 44), and if HC1 is
`smaller than HC2 (step 45), the current is increased
`(step 46). Thus, the emission wavelength can be con-
`trolled to be HC2.
`1. At 273 K,
`‘when i= 10 mA, v=1.720 V and the central frequency,
`HC, =679.2 nm are detected, and when i=20 mA,
`v= 1.825 V and the central frequency, HC, =680.6 nm
`are detected, and these data are substituted in equation
`(1), a value for a of 0.1945336 is obtained.
`.
`2. At 293 K,
`when i=10 mA, v=1.690 v and the central frequency,
`HC=682.7 nm are detected, and when i=20 mA,
`v= 1.800 V andthe central frequency, HC, =684.1 nm
`are detected, and these data are substituted in equation
`(1), a value for a of 0.1945654 is obtained. Therefore,it
`can be seen that a is a constant independent of tempera-
`ture, the value being approximately 0.2. When the equa-
`tion (1) is solved for 8 under the conditions 1 and 2
`above, B=4.707 x 10-4.
`In equation (1), when the standard temperature, Td,
`=273 K, Eg= 1.828580, the temperature, Ti, =333 K
`(60° C.), and the current
`i
`flowing through the
`LED=30 mA,the voltage, v, =1.86 V is obtained
`using the values, a=0.2, and B=4.707 x 10-4. When
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`HCis obtained by an operation process using these data,
`the emission wavelength, HC, is found to be 692.888
`nm. On the other hand, the measured value is 692.900
`nm, and the error (difference between the emission
`wavelength energy obtained by an operation and the
`measured value) is as little as 0.03 meV.
`As can be seen from the above,it is possible to con-
`trol the emission wavelength of an LED by measuring
`the current and voltage of the LED,as well as measur-
`ing the temperature of the LED with a simple apparatus
`without actually measuring an emission spectrum using
`a large and/or expensive apparatus such as a spectrome-
`ter. Thus, according to the present invention,it is possi-
`ble to control the emission spectrum of an LED at low
`cost such that the spectrum control of the present in-
`vention can be used with LED standard light sources,
`LEDlight sources for sensors, optical communication
`equipment, etc.
`It will be understood by those skilled in the art that
`the above described features of the invention are illus-
`trative and not limiting. Variations and modifications
`will be recognized by those skilled in the art as being
`within the scope of the present invention.
`I claim:
`1. An emission spectrum controlling apparatus for at
`least one light emitting diode comprising:
`a temperature measurement means for measuring at
`least one of the temperature of a corresponding one
`of said at least one light emitting diode and the
`temperature in an environment proximateto a cor-
`responding one of said at least one light emitting
`diode;
`a driving power control means for controlling the
`driving power of the at least one light emitting
`diode;
`.
`a computing means for computing an emission wave-
`length of the at least one light emitting diode; and
`emission wavelength control means for controlling
`the driving power control means based on the tem-
`perature information from the temperature mea-
`surement means and driving power information
`from the driving power controlling means.
`2. An emission spectrum controlling apparatus as in
`claim 1, wherein said at least one light emitting diode
`comprises a plurality of light emitting diodes, and said
`at least one temperature measurement means comprises
`a plurality of temperature measurement means.
`3. An emission spectrum controlling apparatus as in
`claim 1, wherein said driving power control means
`comprises at least one of a current measurement and
`contro] means and a voltage measurement and control
`means.
`
`4. An emission spectrum controlling apparatus as in
`claim 3, wherein said emission wavelength control
`Ineans comprises a means for comparing the computed
`emission wavelength from the computing unit with a
`desired emission wavelength, and controlling at least
`oneof said current measurement and control means and
`said voltage measurement and control meansto arrive
`at said desired emission wavelength of said at least one ,
`light emitting diode..
`5. An emission spectrum controlling method for con-
`trolling the emission spectrum ofatleast one light emit-
`ting diode comprising:
`measuring at least one of the temperature of said at
`least one light emitting diode and the temperature
`in an environment proximate to said at least one
`
`6
`
`

`

`5
`light emitting diode with a temperature measure-
`ment means;
`controlling the driving poweroftheat least one light
`emitting diode with a driving power control
`means;
`computing an emission wavelength ofthe at least one
`light emitting diode based on the temperaturein-
`formation from the temperature measurement
`means and driving power information from the
`driving power contro! means; and
`controlling the driving power control meansbased on
`the computed emission wavelength.
`6. An emission spectrum controlling method as in
`claim 5, wherein said at least one light emitting diode
`
`6
`comprises a plurality of light emitting diodes, and said
`at least one temperature measurement means comprises
`a plurality of temperature measurement means.
`7. An emission spectrum controlling method as in
`claim 5, wherein controlling the driving power com-
`prises controlling at least one of current and voltage.
`8. An emission spectrum controlling method as in
`claim 7, further comprising comparing the computed
`emission wavelength with a desired emission wave-
`length and controlling at least one of current and volt-
`age to said at least one light emitting element to achieve
`said desired emission wavelength.
`*
`*
`*
`«x
`*
`
`5,334,916
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`35
`
`65
`
`7
`
`