Introduction

  The human eyes cannot perceive objects that are smaller than a certain size. Also, they cannot recognize light whose intensity is lower than an inherent threshold. We take these limitations into account to determine the number of fluorescent proteins that must accumulate before we can notice any fluorescence, and establish the minimum circular area required for us to perceive any fluorescence.

  Objective

  Investigate concentration required for an amount of fluorescent protein in the E.coli that makes light from E.coli be visible for human.

  Background

  Human has the limits in vision. For our objective, we have to know about the limit of recognizing size of objects in human vision. This limit is called the ‘Minimum visible acuity’. The exact definition of minimum visible acuity is the minimum size of object that the human eyes can discern. In the table 1 Types of visual acuity(reference 1), the value of detection acuity(red box), ~1.0 arc second, is the minimum visible acuity that we take.

  Table : Types of visual acuity ¹

  We assume that the E. coli is in a darkroom for discovering the minimum number of fluorescent protein. In this reason, we use 0.1 lx, the minimum intensity of light that cone cells in human can perceive. ² In fig1 (a), There is a brief picture for minimum visible acuity. We choose that 1 second is the basic time scale, so have to know the number of fluorescent protein per 1 second. By our research, The range of the photon emitted time is wide, from to Although it hasn’t any theoretical reason, we choose the photon emitted time by a fluorescent protein, .³   We will proceed this modeling by using the energy of photons for relating the light and the number of fluorescent protein. One of the unit for light is lm(lumen) and this unit is transformed to J(joule) in ⁴   In fig1 (b), There are two values that we will use.

  Analysis & Result

  For achieving our objective, first we assume that the pictures by E.coli are located at 1 meter from human eyes. From the Background, minimum visible acuity of human is ~1.0 arc second. ~1.0 arc sec, equivalent to , is a very small quantity and therefore can be approximated as . Hence, the smallest object at a distance 1 meter away discernable to the human eye occupies of area. Additional explanation by illustration will follow in fig1 (a). Then, we calculate the number of fluorescent protein per second in the discernable area, . The cone cells in the human retina recognize green, yellow, red, blue emitted by the GFP, YFP, RYP, and CYP. They detect light with intensity greater than 0.1 lux. Therefore, objects must emit light with of luminous flux if they are to be discernable at a meter away. The energy of a photon emitted by a single GFP is . Because it takes about for a fluorescent protein to emit a photon, approximately photons are emitted per second. Thus, the energy emitted by a GFP per second is . In order to satisfy this value, we need