# Applications of Pulsed Light Decontamination Technology in Food Processing: An Overview
## 2 Overview of PL Technology
The light pulses produced are short-lived, but are of high intensity, summing up to around twenty thousand times the sunlight intensity at the sea level [1; 6; 13]. Broad-spectrum pulsed light inactivates microorganisms by a combination of photo-chemical, photo-thermal, and photo-physical effects [16; 17], which are discussed later. Mainly, the lethal effect is owed to the photo-chemical effect whereby light photons of the UV-C wavelengths, notably 253.7 nm, are absorbed by the microbial DNA. Thus, it shows antimicrobial effects alike continuous UV processing [18]. Thereby, similar to continuous UV processing, rules and terms associated with photophysics and photochemistry laws have been applied for PL treatment characterization since its inception. Light-based processes are mainly quantified in terms of the cumulative energy dose imparted during treatment so that the results of PL can be uniformly reported. Typically, the term fluence is used to quantify the radiant energy for treatment. Fluence (F\({}_{\text{o}}\)) is the cumulative radiant energy comprised of all wavelengths per unit area coming from all directions incident on an imaginary infinitesimal sphere with cross-section area dA (unit: J/cm\({}^{2}\)) (Figure 3). The PL inactivation efficacy is dependent upon the delivered fluence [4]. Parameters affecting the fluence (at a particular distance from the lamp) are the pulse width (\(\tau\)), treatment time (t), number of pulses (n), pulse repetition rate (f) [19]. The fluence is mathematically calculated as the product of energy of each pulse, total number of pulses, and pulse frequency as expressed by Equation (1) [20]. \[F_{o}=e\times t\times f \tag{1}\]  
where \(F_{o}\) = fluence (J/cm\({}^{2}\)), \(e\) = energy of one pulse (J/cm\({}^{2}\) per pulse), \(t\) = treatment time (s), and \(f\) = pulse frequency (Hz). Other terms related to PL like fluence rate (W/cm\({}^{2}\)), spectral irradiance (W/(cm\({}^{2}\)-nm)), radiance exposure (J/cm\({}^{2}\)) etc. have been described by [PERSON] and [PERSON] [19]. The researchers tried to introduce the fundamentals of photochemistry and biodosimetry as applicable to continuous UV light wave processing to PL tests. Light energy, as continuous UV, has been used for a long time for disinfection and decontamination in hospitals, laboratories, etc. However, PL application involves a slightly different approach. During PL application, the electrical energy is first concentrated over a long period of time, and then discharged  
Figure 3: Graphical interpretation of fluence. Total cumulative radiant energy incident on the small sphere from all directions per unit cross-section area of the sphere. in the form of short-duration light pulses. These light flashes have more decontamination efficacy than continuous UV light application due to their higher intensity. This may arguably be due to the shorter pulse duration leading to higher impulsive power at equivalent radiant energy. This has been explained to be similar to the concept of 'impulsive force', wherein a short-duration impact force (say a hammer) produces greater impact. Pulsed UV systems can dissipate \(\sim\)35 MW of impulsive power for a fluence level of 3 J/cm\({}^{2}\)[21]. On the contrary, a continuous wave UV system dissipates power in the range of 0.1-1 kW [22]. PL treatment is proven to be more efficient in reducing pathogens like _Escherichia coli_ O157:H7 and _L. monocytogenes_ than continuous UV light irradiation [23]. In a recent study done by [PERSON] et al. [24], results show _E. coli_ inactivation by using pulsed UV-LED irradiation is more effective than continuous UV. By applying the same dose, the inactivation rate of _E. coli_ significantly rose as the duty cycle reduced from 100% to 5% [24]. In addition to a higher decontamination power, high intensity light pulses also penetrate a larger distance through materials than continuous UV light, although both might have the same penetration depth (the distance travelled by the light before its' intensity becomes 1/e of itself) [25; 26].