How can I center the text in my table, so the information is closer in the pdf?

I have programmed the following table in my document, but there is so much space between each column.

I would like to reduce the horizontal space so it looks nicer. Jut like this example.

My code is below and any suggestion(s) are more than welcome.

begin{table}[ht]
        centering
        caption{ Radiometric and photometric units for light intensity characterization. Radiometric units describe the absolute radiant energy, whereas photometric describe brightness as perceived by the human eye. }
        label{tbl:radiometri_photometric}
        begin{center}
            
            begin{tabular}{*{6}{p{.16linewidth}}}
                toprule
                multicolumn{6}{c}{Radiometric} 
                toprule
                multicolumn{2}{p{.33linewidth}}{Symbol} & multicolumn{2}{p{.33linewidth}}{Quantity} & 
                multicolumn{2}{p{.33linewidth}}{Unit} 
                midrule
                multicolumn{2}{l}{$Phi_e$} & multicolumn{2}{l}{Radiant flux} & multicolumn{2}{l}{si{watt}} 
                multicolumn{2}{l}{$E_e$} & multicolumn{2}{l}{Irradiance} & multicolumn{2}{l}{si{wattpersquaremeter}} 
                multicolumn{2}{l}{$I_e$} & multicolumn{2}{l}{Radiant intensity} & multicolumn{2}{l}{si{wattpersteradian}} 
                multicolumn{2}{l}{$L_e$} & multicolumn{2}{l}{Radiance} & multicolumn{2}{l}{si{wattper(squaremetercdotsteradian)}} 
                bottomrule
                
                %toprule
                multicolumn{6}{c}{Photometric} 
                %toprule
                %multicolumn{3}{p{.49linewidth}}{School} & multicolumn{3}{p{.49linewidth}}{State} & 
                %multicolumn{2}{p{.33linewidth}}{Unit} 
                midrule
                multicolumn{2}{l}{$Phi_v$} & multicolumn{2}{l}{Luminous flux} & multicolumn{2}{l}{Lumen (lm)=si{candelacdotsteradian}} 
                multicolumn{2}{l}{$E_v$} & multicolumn{2}{l}{Illuminance} & multicolumn{2}{l}{Lux(lx)=si{candelacdotsteradianpersquaremeter}} 
                multicolumn{2}{l}{$I_v$} & multicolumn{2}{l}{Luminous intensity} & multicolumn{2}{l}{Candela (cd) = si{lmpersteradian}} 
                multicolumn{2}{l}{$L_v$} & multicolumn{2}{l}{Luminance} & multicolumn{2}{l}{Nit (nt) = si{candelapersquaremeter}} 
                bottomrule
            end{tabular}
        end{center}
    end{table}

documentclass{article}
usepackage{array}
usepackage{caption}
usepackage{booktabs}
usepackage{siunitx}

begin{document}


begin{table}[ht]
centering
caption{ Radiometric and photometric units for light intensity characterization. Radiometric units describe the absolute radiant energy, whereas photometric describe brightness as perceived by the human eye. }
label{tbl:radiometri_photometric}
begin{tabular}{@{}lll@{}}
    toprule
    multicolumn{3}{c}{Radiometric} 
    midrule
    Symbol & Quantity & Unit 
    midrule
    $Phi_e$ & Radiant flux & si{watt} 
    $E_e$ & Irradiance & si{wattpersquaremeter} 
    $I_e$ & Radiant intensity & si{wattpersteradian} 
    $L_e$ & Radiance & si{wattper(squaremetercdotsteradian)} 
    midrule
    multicolumn{3}{c}{Photometric} 
    midrule
    $Phi_v$ & Luminous flux & Lumen (lm)=si{candelacdotsteradian} 
    $E_v$ & Illuminance & Lux(lx)=si{candelacdotsteradianpersquaremeter} 
    $I_v$ & Luminous intensity & Candela (cd) = si{lmpersteradian} 
    $L_v$ & Luminance & Nit (nt) = si{candelapersquaremeter} 
    bottomrule
end{tabular}
end{table}


end{document}