focus Subroutine

        subroutine focus(this, spectrum, dict, seqs)

            use random,        only : ranu, seq
            use sim_state_mod, only : state
            use utils,         only : deg2rad
            use vector_class,  only : length
            use tomlf,         only : toml_table, get_value
            use piecewiseMod

            class(photon) :: this
            type(spectrum_t), intent(in) :: spectrum
            type(toml_table), optional, intent(inout) :: dict
            type(seq), optional, intent(inout) :: seqs(2)

            type(vector)  :: targ, dir
            real(kind=wp) :: dist, tmp
            integer       :: cell(3)

            targ = vector(0._wp,0._wp,0._wp)

            this%pos%x = ranu(-state%grid%xmax, state%grid%xmax)
            this%pos%y = ranu(-state%grid%ymax, state%grid%ymax)
            this%pos%z = state%grid%zmax - 1e-8_wp

            dist = length(this%pos)

            dir = (-1._wp)*this%pos / dist
            dir = dir%magnitude()

            this%nxp = dir%x
            this%nyp = dir%y
            this%nzp = dir%z
            this%phi  = atan2(this%nyp, this%nxp)
            this%cosp = cos(this%phi)
            this%sinp = sin(this%phi)
            this%cost = this%nzp
            this%sint = sqrt(1._wp - this%cost**2)

            this%nxp = this%sint * this%cosp
            this%nyp = this%sint * this%sinp
            this%nzp = this%cost

            this%tflag = .false.
            this%bounces = 0
            this%cnts = 0
            this%weight = 1.0_wp
            call spectrum%p%sample(this%wavelength, tmp)

            ! Linear Grid 
            cell = state%grid%get_voxel(this%pos)
            this%xcell = cell(1)
            this%ycell = cell(2)
            this%zcell = cell(3)

        end subroutine focus