specularExponent

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 420 200" xmlns="http://www.w3.org/2000/svg">
  <filter id="diffuseLighting1" x="0" y="0" width="100%" height="100%">
    <feSpecularLighting in="SourceGraphic" specularExponent="1">
      <fePointLight x="60" y="60" z="20" />
    </feSpecularLighting>
  </filter>
  <filter id="diffuseLighting2" x="0" y="0" width="100%" height="100%">
    <feSpecularLighting in="SourceGraphic" specularExponent="5">
      <fePointLight x="60" y="60" z="20" />
    </feSpecularLighting>
  </filter>

  <rect
    x="0"
    y="0"
    width="200"
    height="200"
    style="filter: url(#diffuseLighting1);" />
  <rect
    x="0"
    y="0"
    width="200"
    height="200"
    style="filter: url(#diffuseLighting2); transform: translateX(220px);" />
</svg>