Design of Photosynthetically Active Radiation Sensor

Humans tend to regard plants as passive organisms because of their immobility. But plants are very active and they respond to environment very well. The process called photosynthesis plays a vital role in growth of a plant. In photosynthesis process one of the major requirements would be Sunlight. The whole spectrum of sunlight is not used by plants it is just a part of a band which is in visible region (400nm-700nm). In the other words it is called as Photo synthetically Active Radiation (PAR).


Introduction
Humans tend to regard plants as passive organisms because of their immobility. But plants are very active and they respond to environment very well. The process called photosynthesis plays a vital role in growth of a plant. In photosynthesis process one of the major requirements would be Sunlight. The whole spectrum of sunlight is not used by plants it is just a part of a band which is in visible region (400nm-700nm). In the other words it is called as Photo synthetically Active Radiation (PAR).
Plants very particularly respond to red and blue light and reflects green light [1]. The amount of PAR utilized, varies from plant to plant. Due to variations in spectral and temporal distribution of radiation which is having strong influence on plants, it is quite necessary to measure the quantity of radiation. The response to light conditions is reflected as variation in the rate of evaporation, transpiration and water uptake, their intermodal length, leaf size, orientation and chloroplast density and in turn decides the their flowering time and productivity time [2]. PAR can also be used to study light attenuation in atmosphere, Photo-inhibition, submerged aquatic vegetation, Plant productivity under test environment like green house and Photosynthetic Photon Flux Density (PPFD) under canopies and under water [3].
Measuring PAR involves measure of photon flux density. It is either measured in terms of Energy or by using Photo biological unit in terms of mole. When it comes to unit of measurement, it is sometime relative such as Lux, LUMEN and sometimes radiometric measurement such as watts, joules etc. Yet there is one more form of measurement depending on the number of photons incident, and is called as photo biological unit, µmols-1m-2. This paper deals with design and optimizing of a PAR sensor. Construction of PAR Sensor, mechanical design and various diffusers used for cosine correction is studied to optimize the performance of the sensor. Various performance parameters such as sensitivity, stability of the sensor also is detailed [4]. During terrestrial conditions, PAR value ranges from 0 to 3000µmols -1 m -2 . PAR at night is zero and during sunny days in summer the value of PAR would reach 2000 to 3000µmols -1 m -2 . Heart of PAR sensor is a high sensitivity silicon photo detector which is integrated with specially designed pass band optical filter whose pass band is 400nm to 700nm, which provides uniform sensitivity in that band of wavelengths. Aluminium housing is designed for packaging of the sensor. Figure 1 shows construction of PAR sensor. The characteristics of each element used with PAR Sensor are explained below

Silicon photo detector
Silicon Photo diodes are used in photovoltaic mode, wherein, the incoming radiation is converted to the form of photo current proportional to the number of input photon. Silicon photo diode used in PAR sensor has high current responsively in the solar spectrum, very low dark current, better linearity and minimal change in photo-response characteristics within the operating temperature range [5].
The salient features of Silicon Photo Diode is given below.
The responsivity of the Silicon Photon Diode used in the sensor is shown in Figure 2

Optical pass band filter
Optical filter used in PAR sensor is a special designed pass band glass filter. The pass band wavelength is 400nm-700nm. Dichroic type of optical filter is used. The response of the optical filter is given Figure 3 having maximum transmission of 95% from 400nm to 680nm and with band edge rejection of 2%. Active cosine correction: The sensor is made to align normal to the incident radiation each and every time before doing measurement which is highly time consuming and laborious.
Passive cosine correction: An optical diffuser of specific shaped geometry is used to achieve cosine correction. Selection of material and geometry are crucial in the design of the optical diffuser. Generally, there are two types of diffusers transmissive and reflective. The important properties of the diffuser that decide the ultimate performance are: Light transmission, Diffusion, and Efficiency. To get the greatest possible light transmission and greatest possible diffusion the efficiency should be high [6]. Diffuser Efficiency is obtained by the product of transmission with the diffusion parameters. Similarly 100% true response depends on the height of diffuser. Experimentally it is observed that, when the diffuser vertical edge is raised by 0.75mm it shows 100% true response. So we use acrylic diffuser with thickness 6mm which is raised by 0.75mm vertically. In the present construction an acrylic diffuser with thickness 6mm±10µm, diameter 12mm and an efficiency value of () is adopted.

Diffuser and cosine correction
The sensor should be cosine corrected in order to ensure accurate measurement from the sensor over FOV ±83° to obtain a true response. Lambert's cosine law states that radiant intensity observed at a surface is directly proportional to the cosine of the angle of the incident radiation. Errors occur when the incident radiation is not normal to the surface of the sensor. To avoid these errors cosine corrected heads is used in the sensor. The output of the sensor is strongly dependent on the angle of incident radiation. So, cosine correction is quite necessary.
There are two types of cosine correction 1.
Passive cosine correction

Housing
Housing of PAR sensor is of utmost element due to the facts that it should provide herm city of 5×10-7, moisture resistance,

Processing Electronics
Processing electronics plays an important role in reading output from the sensor. Since Photo biological unit measuring number of photons in the 400nm-700nm range received by plant surface for Photosynthesis at a specified time is µmol/m2/s. The novel approach/design has been carried out, right from converting photodiode current into voltage and converting it into digital value to arrive at higher dynamic range under cloud and clear sky condition. Versatile field test friendly wireless system is incorporated to read the value from sensor via Bluetooth module [7]. The salient block diagram of the processing electronics and its realization is shown in Figure 4. The function of each component is discussed below.

Trans-impedance amplifier
Trans-impedance amplifier (I-V converter) is basically used to convert current into voltage. Since we are using op-amp in inverting mode the gain is just the feedback resistance used. Deciding what should be the gain of the amplifier completely depends on the ranges required for the user. We use four different ranges (20,200,2000,20000) in the device so there will be four different gains in the amplifier stage. Since the ranges are one order higher consecutively the gain will also be of one order difference consecutively. The gain selected would be (1M,100k,10k,1k). A zero offset drift and rail to rail opamp is required for transimpedance amplifier stage. AD8630 is the opamp used in the present design.

Multiplexer
Multiplexer is used for making gain selection in transimpedance amplifier. It is used in feedback for selection of four different gains, so 4:1 multiplexer is used. Multiplexer has two select lines for selecting four different gains, which is controlled by a microcontroller. Two digital pins of microcontroller is used as select lines to the multiplexer [8]. HC4052 is the multiplexer used. A two 4:1 Mix with same select lines is preferred hence HC4052 is used.

Microcontroller
Microcontroller is a heart of processing electronics. Microcontroller is necessary to make majority of the operations like selecting the gain of the trans-impedance amplifier by providing signal to the select lines of the multiplexer, retrieving date and time from the RTC, Data logging into SD card, communicating to the mobile handset via Bluetooth and converting analog data output of trans-impedance to digital value. Microcontroller used here is ATmega328p which is easy to be programmed using Adriano software. The main specifications of AT mega 328p is it has 6 analog pins, compatible with many integrated circuits, ease of programming [9].

RTC
RTC (Real Time Clock) is used get date and time and it used for data logging purpose. RTC communicates to the micro controller serially. DS3231 is the RTC module which is used here. Microcontroller communicates to RTC using I2C protocol.

SD card
SD card is used for data logging purpose and it communicates to the Microcontroller serially. 4 GB Micro SD card can be used and it is the maximum expandable memory limit. Microcontroller communicates to SD card module using SPI interface protocol [10].

16MHz crystal
16 MHz crystal is used as a secondary clock for Microcontroller. ATmega328p is only compatible with 16MHz crystal hence 16MHZ crystal is used.

Bluetooth module
HC-05 Bluetooth module is used communicate to a Mobile handset to a microcontroller. To make system low powered and wireless we use Bluetooth module and communicate wirelessly when necessary. It serially communicates with microcontroller and in return communicates with the mobile handset.

Mobile handset
Mobile handset place a very crucial role, because it is the external controller used for Microcontroller. It is also used to display PAR output, which makes the system simple and compact. An Android application is build and used to communicate to the microcontroller using Bluetooth. MIT app inventor 2 is the innovative tool to build an android application and the flow chart of the logic used in the app is given below. There are two phase in Android app developing, one is designing the page of the android application, and the other one is designing the logic to be used in android application. Figure 5 shows the complete PAR sensor system which consists of Processing electronics, Mobile handset and sensor.
illuminated white LED source with constant input of 40mA is shown in the test setup The PAR sensor is mounted to a Thor labs rotary stage using Thor labs jig. LED light source is kept in axis with PAR sensor at a distance of 35 cm. The Thor labs rotary stage is mounted to a Huber table. The current from the sensor is measured and compared with NIST calibrated Licor sensor. Cosine Corrected PAR sensor developed was comparable with the standard Licor sensor Figure  6. The results of studies in each sensor evaluation is given below.    The above figure shows Responsivity of PAR sensors with various diffusers. PAR Sensor with acrylic diffuser is close to the cosine curve which means cosine correction need to be done correctly by raising little more the vertical height of the diffuser Figure 8.

Cosine corrected acrylic diffused PAR sensor
The above figure shows the cosine corrected PAR sensor's spatial response which almost co-insides with the cosine curve Figure 9.

Transmittance evaluation of diffusers
Transmittance evaluation is carried out using Spectrometer Perkinenseiver L950. The above figure shows the transmittance comparison between acrylic and Teflon diffuser. The figure depicts that Acrylic has high transmittance than Teflon and most important thing as discussed is transmittance in acrylic is most uniform, so use of acrylic diffuser in PAR sensor is more reliable to obtain true response Figure 10. PAR sensor developed with the NIST calibrated Licor Quantum sensor [12]. The below figure shows that the PAR sensor output is 9.25 times the standard Licor Quantum Sensor Figure 11. The calibration multiplier for the sensor built would be equal to (-142.6/9.25=-15.41) -15.41.

Figure 11: Transmittance comparison between
Acrylic and Teflon. The field test comparison shows that the PAR sensor output has just a +/-4% error when compared with standard Licor Quantum sensor which is acceptable limit because the standard Licor Quantum sensor has a acceptable deviation in output of +/-5% which is higher than the PAR sensor developed. Field test is carried out to compare developed PAR sensor with NIST calibrated licor sensor over measurement of 7 hours 40mins which is shown in Figure 12&13.
Detector: High stability photo voltaic detector.

Conclusion
The Novel PAR sensor is developed and validated against NIST calibrated Licor sensor. Field test is carried out and is comparable with NIST calibrated Licor sensor. The processing electronics designed can be customized by the user by adding more peripherals like GPS and GSM for knowing the exact location of the measurement made. There is still a future scope in increasing the dynamic range of a data logger. Making the system wireless has made it a low power and compact and the advantage is it becomes a standalone display unit which is basically a mobile handset, thus this makes the system first of it kind and it would greatly help in taking necessary measures for increasing the agricultural productivity.