LED CONTINUOUS CONSTANT IRRADIANCE WITH TEMPERATURE VARIATION

Abstract

A temperature-stabilized LED irradiance system is provided. The system includes an LED. A temperature sensor is disposed to sense a temperature proximate the LED. Circuitry coupled to the temperature sensor and the LED, is configured to adjust power to the LED based on the sensed temperature.

Claims

1. A temperature-stabilized LED irradiance system for a wet chemistry analyzer comprising: an LED; a temperature sensor disposed to sense a temperature proximate the LED; and circuitry coupled to the temperature sensor and the LED, wherein the circuitry is configured to calculate a power adjustment based on the sensed temperature, and further configured to adjust power to the LED, based on the calculated power adjustment such that a substantially constant irradiance is maintained by the LED.

2. The temperature-stabilized LED irradiance system of claim 1, wherein the LED is an illumination source in a colorimetric analyzer.

3. The temperature-stabilized LED irradiance system of claim 1, wherein the irradiance of the LED is kept constant by continuously controlling the LED power.

4. The temperature-stabilized LED irradiance system of claim 1, wherein power is adjusted by controlling current to the LED.

5. The temperature-stabilized LED irradiance system of claim 1, wherein the temperature sensor is heatsinked to the LED.

6. The temperature-stabilized LED irradiance system of claim 1, wherein the temperature sensor is disposed within the LED.

7. The temperature-stabilized LED irradiance system of claim 1, wherein the circuitry comprises: a digital circuitry component and an analog circuitry component.

8. A light emitting diode (LED) for a wet chemistry analyzer, the LED comprising: a temperature sensor configured to sense a temperature within the LED; feedback circuitry coupled to the temperature sensor, the circuitry configured to, based on the sensed temperature, calculate a power adjustment required to maintain an irradiance of the LED; and; diver circuitry configured to adjust a power provided to the LED based on the calculated power adjustment.

9. The LED of claim 8, wherein the temperature sensor is heatsinked to a p-n junction of the LED.

10. The LED of claim 8, wherein the temperature sensor comprises a thermocouple.

11. The LED of claim 8, wherein the temperature sensor comprises a resistance temperature detector.

12. The LED of claim 8, wherein the circuitry comprises: a digital circuitry component and an analog circuitry component.

13. The LED of claim 8, wherein an irradiance of the LED is kept constant by continuously adjusting the power provided to the LED.

14. The LED of claim 8, wherein the LED is configured for providing illumination in a wet chemistry analyzer.

15. A method of operating an LED within a wet chemistry analyzer, the method comprising: monitoring a temperature signal of a temperature sensor disposed to sense a temperature proximate the LED; calculating a power adjustment for an LED based on the temperature signal; and adjusting a power provided to the LED based on the calculated power adjustment; repeating the steps of monitoring the temperature signal, calculating the power adjustment, and adjusting the power at a frequency sufficient to substantially maintain an irradiance level emitted by the LED.

16. The method of claim 15, wherein adjusting comprises adjusting a current provided to the LED.

17. The method of claim 15, wherein the temperature sensor is heatsinked to the LED.

18. The method of claim 15, wherein the calculating comprises an input indicative of a temperature-irradiance constant specific to the LED.

19. The method of claim 15, wherein the calculating and adjusting are completed by controller circuitry.

20. The method of claim 15, wherein the calculating comprises determining a target power to maintain a given irradiance at the sensed temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 illustrates a portion of a colorimetric analyzer in accordance with an embodiment of the present invention.

[0008] FIG. 2A is a chart illustrating irradiance versus temperature in accordance with an embodiment of the present invention.

[0009] FIG. 2B is a chart illustrating irradiance versus current in accordance with an embodiment of the present invention.

[0010] FIGS. 3A-B illustrate a LED circuits in accordance with an embodiment of the present invention.

[0011] FIG. 4 illustrates a flow diagram of a method for LED control in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0012] The present disclosure relates to a light emitting diode (LED) that maintains a constant irradiance during temperature variations. A variety of colorimetric wet chemistry analyzers employ LEDs in order to generate the light required for colorimetric measurements. LED's utilize electron flow across a p-n junction to generate light. When driven at a constant current, the irradiance of an LED is a function of the temperature of the environment of the LED, i.e., the temperature of the LED substrate (die). This is especially so as the LED emits light closer to the infra-red light (IR) spectrum. In many situations, these temperature-based variations in the irradiance may be acceptable to a user of the LED. However, in some applications, such as colorimetric analyzers, such variations can cause measurement errors.

[0013] When an LED is used as the light source for a colorimetric analyzer, the light intensity of the light source changes with the temperature of the environment where the analyzer is placed. The light absorbance A, is set forth in Equation 1, below:

A=−log 10(I/I.sup.0)   Equation 1

[0014] Where I is the measured light intensity when color has been developed after reactions, and I.sup.0 is the measured light intensity before the color is developed. During operation of the wet chemistry analyzer, these two measurements of light intensity occur at different times. If the ambient temperature changes during the process and if the LED is driven at a constant electric current, the calculation of the measured (I/I.sup.0) will include an error caused by the irradiance dependency on temperature.

[0015] FIG. 1 illustrates a portion of a colorimetric analyzer in accordance with an embodiment of the present invention. Colorimeter analyzer 100 may be useful to analyze a liquid 108 located in photometric cell 106 by passing an illumination 104 through liquid 108. LED 102 generates illumination 104 which passes through liquid 108 and is detected by a photo sensor 110. Photo sensor 110 may be any sensor that detects a characteristic of light, including, but not limited to, a colorimeter, spectroradiometer, spectrophotometer, or densitometer.

[0016] In order for multiple measurements taken by colorimeter analyzer 100 to be compared, illumination 104 must be consistent. If there is variation in illumination 104 between measurements, the variation must be adjusted for or the comparison between measurements will be inaccurate. A problem with LED's is illustrated in FIG. 2A. Chart 200 has an X-axis 202 representing temperature and Y-axis 204 representing irradiance. Trend line 206 illustrates how as temperature of an LED increases, the irradiance of the LED decreases.

[0017] One solution to this problem would be to, if temperature changes, drain liquid 108 from photometric cell 106 and measure un-obstructed illumination 104, as a control value. Then, while maintaining the temperature, refill photometric cell 106 with liquid 108 and project illumination 104 through liquid 108 and measure an experimental value. The experimental value can then be adjusted by the control value. This is a viable solution but comes with the cost of draining-refilling and also increased likelihood of errors during adjusting measured values based on varying illumination 104 values.

[0018] A better solution is to, as temperature changes, ensure that the illumination 104 generated by LED 102 stays constant. Irradiance of an LED is determined by more than just temperature. FIG. 2B is a chart illustrating irradiance versus current in accordance with an embodiment of the present invention. Chart 250 has an X-axis 252 representing current and a Y-axis 254 representing irradiance. Trend line 256 illustrates how as current supplied to an LED increase, the irradiance of the LED increases. In order for an LED to maintain a constant irradiance in a varying temperature environment, it is possible to vary the current supplied to the LED. For example, as temperature increases, current will also have to increase to maintain a constant irradiance.

[0019] FIG. 3A illustrates an LED circuit in accordance with an embodiment of the present invention. LED circuit 300 comprises an LED 302, LED driver circuitry 304, feedback circuitry 308 and temperature sensor 306. Temperature sensor 306 is disposed to sense the temperature proximate LED 302. Temperature sensor 306 can be any suitable structure that has an electrical characteristic that varies with temperature. Examples, include a resistance temperature detector (RTD), a thermocouple, a thermistor, etc. Temperature sensor 306, in one embodiment, may be coupled to the LED 302 die/substrate utilizing a heatsink.

[0020] Temperature sensor 306 is coupled to feedback circuitry 308, which provides a signal to LED driver circuitry 304. Feedback circuitry 308 and/or driver circuitry 304 can be embodied in any combination of digital or analog circuitry. For example, feedback circuitry 308 could be embodied using a microcontroller having an analog to digital converter that receives a signal from temperature sensor 306 and provides a digital indication of temperature. This digital indication can then be used by LED driver circuitry 304 to compute and adjust the power provided to LED 302, thereby controlling irradiance based on the measured temperature. In one embodiment, LED driver circuitry 304 and feedback circuitry 308 are components of a single circuitry.

[0021] While the embodiment shown in FIG. 3A illustrates temperature sensor 306 separate from LED 302, it is expressly contemplated that embodiments can be practiced where the temperature sensor is a part of LED 302 or disposed within the LED package. FIG. 3B illustrates an embodiment, wherein LED driver circuitry 304, feedback circuitry 308 and temperature sensor 306 are disposed within LED 302. However, any of these components may be separate from LED 302.

[0022] FIG. 4 illustrates a flow diagram of a method for LED control in accordance with an embodiment of the present invention. Irradiance stabilization 400 comprises three steps. At block 402, a temperature is measured proximate the LED. In one embodiment, the temperature measurement is taken within the LED.

[0023] At block 404, a power adjustment is calculated. The calculation of power adjustment uses the temperature measured in block 402 as an input. The calculation, in one embodiment, includes temperature vs irradiance data from the manufacturer. In another embodiment, the calculation comprises a temp-radiance constant multiplied by the measured temperature.

[0024] At block 406, the power to the LED is adjusted utilizing the calculation from block 404. The adjustment may be made can be by circuitry, embodied in any combination of digital and/or analog circuitry. After the power supplied to the LED is adjusted, the cycle may repeat and the power supplied constantly adjusted to compensate for any change in temperature.

[0025] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.