deletions | additions       diff --git a/Perspective.tex b/Perspective.tex  index c1fc876..88a8511 100644  --- a/Perspective.tex  +++ b/Perspective.tex 
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In order to test LEDs, a design must consider a variety of factors. The first is how to present interference between the lights to ensure that the status of one light does not impact the test status of the next. The information on this topic can be found in Section \ref{sec:Interference}. One must also consider how to detect the presence of lights in the machine. The theory behind this is described in Section \ref{sec:Presence}. The way in which light is sensed (i.e. which photosensor to use) is also a key component of the design. The different types of photosensors are described in Section \ref{sec:Photothings}. Finally, we can examine an existing solution used in the industry to gather some ideas about how best to combine these components, and what standard practice is for a machine like this. Information regarding existing machines can be found in Section \ref{sec:Market_Survey}.   \subsection{Preventing Interference}  The design team discovered during research that industrial strength blackout curtains were manufactured for use in laser light labs and test labs. In both cases, light is being evaluated and the measurements made need to be accurate. Given that evaluating the functionality of the candlelights requires the same accurate measurement of light, some kind of equivalent of blackout curtains must be included in the machine. Furthermore, the lights within the machine must not interfere with each other once they are all switched on and therefore a blackout wall between lights is required. Blackout curtains function by having a backing made of multi-layered, tightly woven fabric which blocks the light\cite{SteelGuard}. For the purposes of this machine, panelling of wood or opaque plastic can serve this function.  \label{sec:Interference}  \subsection{Presence Detection}  There are two instances during the operation of the machine where the presence of an object must be detected for operation to proceed correctly. The first is before the operation begins, where it must be ensured that the tray has been seated correctly. The second is when a light is to be tested, and a distinction must be made between an absent light and a Type 2 failure. To this end, several devices can be used to detect the presence of an object.  \label{sec:Presence}  \subsubsection{Breakbeam Sensors}  Break-beam sensors also referred to as opposed mode photoelectric sensors have a very simple method of operation. The premise (as depicted in [INSERT FIGURE]) is that an emitter is placed across from a receiver. The emitter directs a beam of light towards the receiver, and an object is detected when this beam is broken. Break-beam sensors are very reliable when it comes to detecting opaque objects, perform well in the presence of other light sources and have a long sensing range \cite{BannerEngineering}. These sensors are often used for applications where counting is required, though could be used to detect the presence of the tray and the candlelights it contains.  \subsubsection{Reflective Ultrasonic}  Ultrasonic sensors [INSERT FIGURE] use ultrasonic waves to detect a wide variety of targets in many environmental conditions, and are particularly useful for detecting transparent objects\cite{Tektron}. However, they must be aligned very precisely with the desired surface and have a minimum sensing distance as there is often a blind spot directly in front of the device, which is an issue for the small scale on which the candlelight’s presence will need to be determined. Furthermore, the sensors must be placed a minimum distance (sometimes up to 1.5 m) away from each other to prevent interference between sensors\cite{RockwellAutomation}. This is also problematic given that the design calls for nine sensors spaced close together. As such, reflective ultrasonic sensors are impractical for for the detection of candlelights and/or the tray in this application. 
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\subsubsection{Phototransistors}  Phototransistors (Figure 4.3.1) [INSERT FIGURE]  are typically bipolar NPN devices with larger base and collector areas optimized to work when exposed to light. While all transistors have some photo-effects, phototransistors generally also have a casing designed to enhance light exposure. Phototransistors function in the same general manner as regular transistors except that they do not have a base terminal connected to a voltage source. Instead, the base is exposed to light, which allows the formation of hole-electron pairs and in turn the flow of current. This flow of current is converted into voltage, which can then be used to transmit a signal to the microcontroller for analysis. Phototransistors typically produce higher current than photodiodes and work with a most visible and near infrared light sources. While these features are advantageous, phototransistors are particularly vulnerable to electric surges\cite{IHSGlobalSpecElectronics}. \subsubsection{Photoresistors}  Photoresistors are constructed out of light sensitive that becomes more electrically conductive with the addition of incident radiation\cite{BannerEngineering}. This decrease in resistance results in a lower voltage across the photoresistor. This change in voltage can be converted to a signal for the PIC microcontroller to use. The primary advantage to photoresistors is that they are generally very inexpensive and have wide sensitivity to visible light. This feature is useful for colour detection. However, they also have a slow and nonlinear response to light, which can make their signal more difficult to interpret. 
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\subsection{Existing Solutions}  \label{sec:Market_Survey}  There are several different machines which are produced for the purpose of testing LEDs in the industry. These devices includ the ones shown in Figure [INSERT FIGURE] below. These devices are primarily used to test the lifetime of LEDs on mass and under a number of different operating conditions. While the technology of most of these machines is beyond the scope of this project, we can examine one machine from Chroma which provides us with some indication of how LEDs are tested in industry\cite{Chroma}.    The 58158-SC LED Lighting In-Line test system [INSERT FIGURE] helps us examine both the old and the new way of testing LED lights in industry. The first important thing to note is that the lights move under a single sensor. This is optimized for the assembly line, and helps minimize the number of moving parts. The second thing to note is the high speed at which this system checks lights. It can test up to 10 000 lights per day. This demonstrates the importance of efficiency and speed the systems used in industry. Finally, the light testing machine relies on a luxmeter - essentially surrounding the light with sensors and measuring the overall brightness\cite{Chroma}. These different features inform the design team about the needs and expectations of the industry, and influenced the designs considered as a solution to this problem.  \subsection{Preventing Interference}  \label{sec:Interference}